So I solved it when I took the advice to delete the student and reenter his 1098T info for this year. At the end, that brought me to a screen asking if he'd gotten these credits in past years. At the bottom of the screen, a "4" was pre-entered for the number of years I've claimed the credit. When I went back through my tax forms for prior years, I realized I claimed this credit for 2016, when he started college in September. So that means this would be the fifth year and you only get the credit for four years.
TurboTax doesn't explain any of this, and I don't even know how it knows that I've already claimed him for four, when it doesn't know other things, like whether I filed a 2019 1098T for him. (I mean, it really should have cumulative records, but it doesn't seem to.)
So, problem solved. Maybe this is the situation some other people on this thread are finding themselves in.
