Why can’t I claim the American Opportunity Credit again this year? It would only be the second time claiming it

You can claim the credit if you paid for anything.  Did you have a 1099-Q or scholarship that covered it all?

Maybe you did it wrong last year. 

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 full time unmarried student, under age 24, even if you don't qualify as a dependent, 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. 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.

Reference: Line 7 instructions for form 8863. https://www.irs.gov/instructions/i8863

If you are eligible, you have answered  something wrong, in the interview. But, a lot of people are just not eligible.

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

See Also:

 https://www.irs.gov/Individuals/AOTC