Did you take a disaster distribution or a Coronavirus-Related Distribution in 2019 or 2020 from an employer plan and chose to include the income in equal parts over three years? That would cause TurboTax to include on 2021 Form 1040 line 5b 1/3 of the original distribution. That's the only way I know of that you could get an amount on line 5b without a a 2021 Form 1099-R being present.
