Q:

How to encourage unbiased discussion on this site?

To my best of knowledge, this site encourages unbiased discussion about politics. We cover facts but not speculation. Please correct me if I'm wrong.
We are encouraged to support our answer with facts and reliable references. The United States White House government, running the strongest economy in the world is a reliable reference.
Unfortunately, some users are probably not aware of our rules. For instance, my answer received downvotes for no reason:

Mexico paying for the wall by tariffs is not my invention, it's absolutely endorsed by the White House and the president himself. Being the president, he speaks on behalf of the country. The Trump administration consistently insist the wall will be indirectly paid by Mexico tariffs. The view that the wall will pay off is official and reliable. Neither the other party or the media have the rights to speak for the country. They don't represent the country, the Trump administration does. Trump's views == US's views.
We have the freedom to speak for ourself. However, anything but White House views are unofficial and thus should be considered as personal speculation.
I suspect I'm getting downvotes because of people were not aware of our rules. I attempted to cover facts in my answer, and then explained it with a business example.
How should I improve my answer to encourage unbiased and more meaningful conversation? How should I edit my answer to make our readers understand facts?

A:

It is understandable when some users do not believe that Donald Trump is a reliable source, because he has repeatedly been caught making false statements in the past. That does not mean that everything Trump said is automatically wrong, but it also shows that not every statement he makes should be believed without any skepticism.
Further, the answer claims that the White House made a certain statement at some point in the past, but does not provide a proper reference to a specific statement. Such a reference could, for example, be a link to a press release on whitehouse.gov, a video from a press conference or at least a second hand account in form of a news article. If the author wants to convince the audience that they indeed did not "imagine things", then it should be possible to provide such a reference which proves that such a statement was indeed made.
Another way to improve this answer could be to add more details about what changes the US government is planning to make at their tariff system in order to improve tariff revenue from Mexico and a back-of-the-envelope calculation which shows that it can indeed cover the cost. These numbers should also be backed by references to reliable sources to show that they are realistic.

A:

Let me break this down as to where you went wrong and how you can get it right in the future

Mexico paying for the wall by tariffs is not my invention, it's absolutely endorsed by the White House and the president himself. Being the president, he speaks on behalf of the country. The Trump administration consistently insist the wall will be indirectly paid by Mexico tariffs. The view that the wall will pay off is official and reliable. Neither the other party or the media have the rights to speak for the country. They don't represent the country, the Trump administration does. Trump's views == US's views.

You're taking what a politician says (never mind that we're talking Donald "Trade Wars Are Easy To Win" Trump) at face value. At best, this is spin. At worst, it's an outright lie. This is a bipartisan tactic

However, anything but White House views are unofficial and thus should be considered as personal speculation.

Your mistake was you didn't start off with "The White House asserted..." You then launched your own speculation

The US is going to charge Mexico's tariffs for the wall. 

Er, no. Tariffs are a tax paid by end consumers. The tariffs in question are for goods coming into the US. So the US is taxing its own people to pay for the wall? That doesn't sound like Mexico is paying for it at all. And Politico shoots this idea down

The funds aren't likely to be coming from tariff revenue. Most tariffs between the U.S. and Mexico were waived nearly 25 years ago through the original free-trade agreement. The new pact keeps most of those tariff reductions in place, which means not much money will be flowing into the Treasury.

So how can you fix this? I can think of a few ways

Make it clear that the White House is asserting something, instead of saying that it's unequivocal truth. Quoting people is always acceptable but be sure that you're not stating it's truth and then proceeding as if that were unquestionable.
Acknowledge the contradiction between Trump's prior campaign promises and the current position (you need to be able to do Devil's Advocacy on Politics.SE)
Add some quotes and links to back your position up. People will still downvote what they disagree with, but at least it won't be due to content

A:

Let's break down what you are claiming here:

Unfortunately, some users are probably not aware of our rules. For instance, my answer received downvotes for no reason:

Plenty of reasons were offered. You don't seem to agree with them. However, that's not the same as "for no reason."

Mexico paying for the wall by tariffs is not my invention, it's absolutely endorsed by the White House and the president himself. Being the president, he speaks on behalf of the country. 

I have no issue with this characterization.

The Trump administration consistently insist the wall will be indirectly paid by Mexico tariffs. 

Not true. Trump, early in his presidential campaign, sent a memo to the Washington Post explaining how he would force Mexico to make a single multi-billion dollar payment to fund the wall.  I already posted that link in my own answer to that question.

The view that the wall will pay off is official and reliable. Neither the other party or the media have the rights to speak for the country. They don't represent the country, the Trump administration does. Trump's views == US's views.

There is nothing to suggest that it is "reliable." It might be the official stance, but there has been nothing offered that, when held up to objective or expert analysis, suggests that this claim is any more than empty political rhetoric.  Stating that this rhetoric is not accurate in no way claims to speak for Trump, or for the nation. And while Trump is the official leader of the nation, in no way does his opinion become the opinion of each and every citizen.  So this argument is really rather meaningless to assessing the validity of a claim.
You state that this is true, but you base this entirely on the person being in charge saying it was so. You offer no evidence to back up the assertion other than "Trump says so."
This is a logical fallacy known as "Appeal to Authority," and in this case, you use a literal authority, as opposed to an expert authority. This might even be the "appeal to false authority."

Description: Insisting that a claim is true simply because a valid authority or expert on the issue said it was true, without any other supporting evidence offered. Also see the appeal to false authority.

Logially Fallacious: Appeal to Authority
Back to your question - 

We have the freedom to speak for ourself. However, anything but White House views are unofficial and thus should be considered as personal speculation.

No, because people saying things contrary to White House views are not claiming they are the official stance of the government. Whether they are speculative or not would be based on supporting arguments, examples, or citation of facts. The White House saying something does not make it factual. 
Anything stated without strong logical arguments and sourcing is just speculation. Whether the source is the official White House stance has nothing to do with the validity of anyone's statements or views.
Richard Nixon claimed he had nothing to do with the Watergate break in that forced him to resign. He was the President. The White House proclaimed this to be so. That did not make it true and factual. In fact, there were recordings of him that absolutely proved he was lying. There is nothing magical about being elected President that makes you stop lying, or transforms your lies into nuggets of truth. Your insistence that what he says is true because of who he is and the office he holds defies logic and what we know.
Ronald Reagan famously stated that he knew nothing about the sales of arms to Iran in the Iran Contra scandal. It later came out that he personally approved those sales. He admitted to lying about it to the American people. How is this possible if what a President says is, by default, the factual truth?
Saddam Hussein and his official government news agency that made statements for the nation were universally laughed at (Google "Baghdad Bob") for the absurd lies they tell.
Kim Jon Un and his predecessors and their government news agency are known for their outrageous lies and propaganda - in part, they have to lie in order to keep control of their people, who live in abject misery.
There is nothing about being the head of a nation, or having official national pronouncements that automatically conveys the power of truth.
We know that Trump is capable of making misstatements and out and out lies and fabrications.  

In the first nine months of his presidency, Trump made 1,318 false or misleading claims, an average of five a day. But in the seven weeks leading up the midterm elections, the president made 1,419 false or misleading claims — an average of 30 a day.

WP Fact Checker:President Trump has made 6,420 false or misleading claims over 649 days 
His press secretary, Sarah Sanders, it infamous for her willingness to make bald-faced lies as the official spokesperson for the White House.
Politifact: All False statements involving Sarah Huckabee Sanders
Even more problematic with your stance is when the White House and the President, themselves, make an emphatic statement that, according to you must be true because it is the White House and the President. And then they change their statement many times, over time, until they are later fully contradicting their original stance (Trump Tower meetings with Russian officials, and who knew about it.  Payments made by Cohen to women claiming to have slept with Trump, whether they ever happened, whether Trump knew about it. How Mexico was going to be forced to pay for the wall.).  If the original statements must be true because it's the White House, and then they make statements that fully contradict that "truth," but they are still the White House making those statements, how can you have two completely contradicting and mutually exclusive stances that must be true?
This is why we don't just stop at "this person said so, so it is."

I suspect I'm getting downvotes because of people were not aware of our rules. I attempted to cover facts in my answer, and then explained it with a business example.

Most people are, for the most part, aware of the rules. Nowhere in those rules is it contained that one's favorite political figure is incapable of being wrong or telling lies, and that is the entire argument you made in that question that was down-voted.
FYI, I did not actually down-vote it, myself. It would have been a pointless piling-on after the question was already deleted.

