I got the same error. Converted all non-deductible traditional IRA to a Roth IRA in 2020, but realized that my basis was incorrect from past years on the 8606 forms so Turbo Tax was seeing a different basis from past filings (I did provide the explanation and amounts of correct basis in the software). I am guessing I will need to mail in my return with the amended basis on the 8606 forms.
