Grad students are not eligible for the more generous and partially refundable American Opportunity credit (AOC). You are eligible for the Lifetime Learning Credit (LLC). The LLC is non refundable. That is, it can only be used to offset an actual tax liability.
If you are eligible, you have answered something wrong, in the interview. But, a lot of people are just not eligible. See Also see this TT FAQ https://ttlc.intuit.com/questions/3383321-why-didn-t-i-get-a-credit-or-deduction-for-education-expen...
https://www.irs.gov/credits-deductions/individuals/education-credits-questions-and-answers
https://www.irs.gov/individuals/llc
