Read the IRS letter carefully to see which technicality they are after you for.
The IRS has begun cracking down on schools that send out a 1098-T with box 1 blank. They are not always accepting an amount in box 2, of a 1098-T, as proof that you paid tuition. There was no additional proof required at the time you filed your return. The IRS may just want the proof , now, after the fact. You may only need to prove you actually paid tuition, with school statements and cancelled checks and/or bank or credit card statements.
But, a student, under age 24, is only eligible for the refundable portion (the $1000 on line 17c of form 1040 (2018), of the American Opportunity Credit ( AOC or AOTC) if he supports himself by working, even if he is not a dependent. 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 another common reason for an IRS letter. You may only need to prove, to them, that more than half your support came from earned income.
Also 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?”
For full AOTC rules, see https://www.irs.gov/Individuals/AOTC
