Why do I qualify for the American Opportunity Credit if I am still in school

That's the only time you qualify for it.  If you are a dependent, then you parents qualify for it, instead. You must be a half time or more student and pay tuition in the tax year to qualify.

Or did you mean NOT qualify?

If you are eligible, you have answered  something wrong, in the interview. But, a lot of people are just not eligible. See https://www.irs.gov/Individuals/AOTC

It may only be you are looking in the wrong place. The non-refundable portion shows on line  3 of schedule 3 (line 50 in 2018) and will be blank if you had no tax liability to off set. The refundable portion (up to $1000 will be on line 18c of  form 1040 (line 17 in 2018).

There's a new urban myth among college students that says they can get a $1000 from the government just for filing a tax form. For most of them, they simply aren't eligible. A unmarried  student, under age 24, is only eligible for the refundable portion of the American Opportunity Credit if he supports himself by working. You cannot be supporting yourself on parental support, 529 plans or student loans & grants. You must have actually paid tuition, not had it paid by scholarships & grants.  It is usually best if the parent claims that credit. 

You cannot claim a credit if you are, or can be, claimed as a dependent by someone else. There is a limit of claiming the AOTC a total of four times (4 tax years) per student, whether claimed by the student or his parents. You cannot file as Married Filing separately.

Go thru the interview again. In particular, Edit your personal information at the beginning of the software. When Turbotax (TT) sees from your birth date that you are under 24, it specifically asks if you provided more than half your support with earned income.

Also see this TT FAQ https://ttlc.intuit.com/questions/3383321-why-didn-t-i-get-a-credit-or-deduction-for-education-expenses