How firm are universities about UCAS applications?

So, in my UCAS application I put AP French as a pending grade because I was planning to take the exam in May. However, I just found out that I was never actually registered for the exam by the school, which I thought I was, and although they offer the class they might not let me sit it at my school (no one else is taking the class). I live in the middle of nowhere, so the gas increases the exam price to about $400 dollars. Basically, I don't want to take it. None of my unis require the exam in my conditional offers because I have previous APs. If I don't take the exam, does that mean I need to redo my UCAS application to make it valid and wait for offers again? Or could I just contact the uni I want to go to and tell them about the problem?

If it's not listed as a condition in your conditional offer, then in all likelihood you will not need to take the exam.

However, you should contact the universities with which you hold offers and just check that they're happy with that approach as technically they could object, even if it isn't listed on your conditional offer.