If 2021 TurboTax sees you enter an amount of nondeductible traditional IRA contributions that differs from that recorded in 2020 TurboTax as the amount that passes forward to 2021, it either means that the basis information in your 2020 tax file is erroneous or you had an event such as acquiring basis from a deceased spouse or transferring basis to a spouse in divorce that added or removed basis from your IRAs that requires explanation to the IRS.
If the information in your 2020 tax file is wrong, you need to correct that first, then start your 2021 tax return over. (It's possible that this information carried forward from your 2019 tax file into your 2020 tax file, so you might have to go back even further than 2020 to find such an error.) If there is instead reason to adjust your basis, you'll need to provide an explanation statement. You are not permitted to change an amount carried forward from your 2020 tax file unless you provide explanation.