The AOC is for students who have not earned a bachelor degree yet. It can be used for up to four times during your undergraduate years, but now that you are a grad student, you cannot use it. Instead you can try to use the Lifetime Learning Credit. The LLC is non-refundable---it can reduce your tax owed, but is not added to your refund.
https://ttlc.intuit.com/questions/3262983-who-is-eligible-to-take-the-american-opportunity-tax-credi...
https://ttlc.intuit.com/community/credits-and-deductions/help/what-expenses-qualify-for-the-lifetime...
https://ttlc.intuit.com/questions/3262984-who-is-eligible-for-the-lifetime-learning-credit
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