See   https://www.irs.gov/individuals/education-credits-questions-and-answers Scroll down to “I received a letter from the IRS questioning my AOTC claim. What should I do?”

Read the IRS letter carefully to see which technicality they are after you for.

A student, under age 24, is only eligible for the refundable portion (the $1000 on line 44 of form 1040A [line 68 on form 1040])of the American Opportunity Credit if he supports himself by working. You cannot be supporting yourself on parental support, 529 plans or student loans & grants. You must have actually paid tuition, not had it paid by scholarships & grants.  It is usually best if the parent claims that credit.  The age 24 rule is a common reason for an IRS letter. You may only need to prove, to them,  that more than half your support came from earned income.

The other common requirement is you only need to prove you actually paid tuition, with school statements and cancelled checks and/or bank or credit card statements. The IRS has begun cracking down on schools that send out a 1098-T with box 1 blank. They are no longer accepting an amount in box 2, of a 1098-T, as proof that you paid tuition.