In the 1098-T portion of the interview, check "At least half-time," as indicated on your form 1098-T.
You probably indicated "Full-time" when you entered your student status in the "My info" section of TurboTax, and this is what you should've done. This indicates to the program that you're eligible for the American Opportunity Credit, but there's no checkbox for "Full-time" on the 1098-T.
