The American Opportunity tax Credit (AOTC) could be $0 for several reasons. You may not have a form 1098-T, your may not have been enrolled for more than half-time, you may have used the credit for four years or completed four year of college already and there may be other reasons.
You can see this IRS article to learn more about it:
If you are eligible, you have answered something wrong, in the interview. But, a lot of people are just not eligible.
There's a new urban myth among college students that says they can get a $1000 from the government just for filing a tax form. For most of them, they simply aren't eligible. A full time unmarried student, under age 24, even if you don't qualify as a dependent, is only eligible for the refundable portion 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 usually must have actually paid tuition, not had it paid by scholarships & grants. It is usually best if the parent claims that credit.
You cannot claim a credit if you are, or can be, claimed as a dependent by someone else.