One requirement for the American Opportunity Credit is that it can only be for the first four years of post-secondary education. That's measured by credit earned rather than calendar years. So if you finished your undergrad education, then you wouldn't qualify.
When you advance thru the entry screens in the Education section (Federal Taxes > Deductions & Credits > Education), you'll be asked a question, Have you earned four years of college credit before 2016? That's the box you would check "yes," given what you've explained above. Perhaps you advanced past it.
Also, you might want to double-check that it's actually the AOC that TurboTax is giving you. Both the AOC and the Lifetime Learning Credit (LLC) -- which doesn't require the schooling be the first four years -- appear on the same line on the tax return (line 50 for 1040, and line 33 on 1040A). In addition, there's a refundable portion of the AOC that carries to line 68 of the 1040 (44 on 1040A).
