Q:

Download button on ubuntu.com does not work - do I have to pay?

Can I Download Ubuntu for free or do I have to pay 20 dollars? 
The Option Not now, take me to the download on http://www.ubuntu.com/download/desktop/contribute/?version=15.10&architecture=amd64 (link edited in as example) does not take me to the download page.

A:

Ubuntu is an open-source Operating System and you do not need to pay anything to download/use it. On the download page, it says "Pay what you think it’s worth" and has a pre-filled default contribution amount of US $16 which you may change as per your liking.
If you do not want to contribute in terms of money there is no one forcing you to do so. As you have already noted yourself, there is a link which says "Not now, take me to the download" (for Ubuntu Desktop LTS 64-bit), clicking that should start the download of the iso image.
If the link doesn't work for you for some reason, then you may change all the contribution fields to 0 and the "Pay with PayPal" button would be converted to the "Download" button.

Another option is to navigate through http://releases.ubuntu.com/ and select the appropriate iso image as per your needs.

