Are you using the same TurboTax account as last year?
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Yes, I'm using the same account. But I think I figured out the problem - it's not Turbo Tax's fault. I looked up my returns from 2017-18 (and maybe a little earlier) to see if I could get the number from those, and it wasn't on those forms either. Evidently, in recent years you only had to put your cost basis down for the first year in which you were reporting the installment sale. The rules must have changed ,to require the cost basis to be listed in subsquent years a well. I think the first year I reported this sale must have been before I started using TurboTax, so it wouldn't be "in the system."
Maybe you'll find this helpful if someone else has the same issue. Thanks for responding.