For 2018 return I got REFUNDABLE American Opportunity Credit. Info I put in TT:I'm single, full time student, NOT a dependent, did NOT support myself. something wrong?

If you're over 23 or an orphan, you can get the refundable portion. 

A student, under age 24, is only eligible if he supports himself by working . You cannot be supporting yourself on student loans & grants.   It is usually best if the parent claims the credit rather than the student. He/she does not qualify for the (up to) $1,000 refundable portion of the American Opportunity Credit (AOC) if items 1, 2, and 3 below apply to him. 

1. He was:
.....a. Under age 18 at the end of 2019, or 
......b. Age 18 at the end of 2019 and his earned income was less than one-half of his support, or 
......c. A full-time student over age 18 and under age 24 at the end of 2019 and his earned income was less than one-half of his support .
2. At least one of his parents was alive at the end of 2019.
3. He is not filing a joint return with his spouse for 2019.

If you meet those rules, and TurboTax is giving it to you anyway, you've answered something wrong in the interview.  In particular, the question about half your support from earned income comes up in the personal info section, if your birthday indicates you are under 24. 

If you are a dependent of another taxpayer then they enter the 1098-T on their return.

Based on the information you provided and if you meet the requirements below you would be able to claim the AOC credit. 

To be eligible for AOTC, the student must:

• Be pursuing a degree or other recognized education credential
• Be enrolled at least half time for at least one academic period* beginning in the tax year
• Not have finished the first four years of higher education at the beginning of the tax year
• Not have claimed the AOTC or the former Hope credit for more than four tax years
• Not have a felony drug conviction at the end of the tax year

*Academic Period can be semesters, trimesters, quarters or any other period of study such as a summer school session. The schools determine the academic periods. For schools that use clock or credit hours and do not have academic terms, the payment period may be treated as an academic period.

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What is the American Opportunity Tax Credit-