Academic policy does not let me earn credits from certain past courses. How can I challenge it? I have read the policy below:

No undergraduate courses taken to satisfy prerequisite requirements for admission to a graduate program may be included in the minimum number of units required for that program.

I completed a course at my previous university that is considered a graduate elective that also happens to be an undergraduate class.
My personal course audit from my former university shows this course is considered an elective for graduate students.
I want to earn credit for the course taken at my current university. Since I took it as a graduate student and I am not using it to satisfy pre-requisite requirements to admission (I am three semesters in at my current university), how can I successfully challenge this policy?
(Edit: I misquoted the quotation above.)
Edit: this answer was written based on the original policy quote.
The policy is clear, if the undergraduate course was taken to satisfy pre-requisite requirements then it can be counted.
As you clearly state that it was not needed to satisfy a pre-requisite requirement, and you state clearly that it was an elective then it will not be counted.
So, it looks like you will need to complete some credits to cover that lack.