The American Opportunity Credit can only be used for undergraduate school. If you are in graduate school you cannot use the AOC even if you completed your bachelors degree in less than four years. However, you can explore using the Lifetime Learning Credit or Tuition and Fees.
https://ttlc.intuit.com/questions/3262983-who-is-eligible-to-take-the-american-opportunity-tax-credi...
https://ttlc.intuit.com/questions/1901172-what-education-tax-credits-are-available
https://ttlc.intuit.com/questions/3262984-who-is-eligible-for-the-lifetime-learning-credit
https://ttlc.intuit.com/questions/1901487-what-is-the-tuition-and-fees-deduction
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