I didn't get the american opportunity credit, even though my enrollment status is full-time, I'm over age 24 filing single, & my income is <$90k. What do I do to get it?

Did you enter a 1098-T with an amount in Box 1 (Tuition) that is larger than the amount in Box 5 (Scholarships)?

The AOC is to help you with out of pocket education expenses, so what you enter in the education section has to indicate that you had expenses not covered by scholarships/grants.

Here's more info on American Opportunity Credit. 

You've entered something wrong. Try deleting the 1098-T and re-entering everything.

Thank you both! Yes, the amount in box 1 was $14,000 and change while the amount in the Scholarship box was around $2,000, so there was definitely uncovered cost! I tried re-entering everything manually but that didn't work either. I'll look through the link that you posted and see if there's anything in there. Thanks to both of you!!

Thanks to the link you posted, I was able to find the answer - I am a graduate student, and the tax credit only applies for the first four years of college education, which I already completed in undergrad. Thanks for your help!

Grad students are eligible for the less generous "Lifetime Learning Credit" (LLC).