Same problem for me. If you entered the information for Form 2021 prior to the update on 2/26/21, it appears that TurboTax has a programming error in that it has kept a "ghost" of your prior entry for Column (b) in the worksheet even though Column (b) is not used in 2020. TurboTax then adds that value with the values for Column (a), (c) and (d) and presto, you have an error. You can fix it by editing the worksheet that pops up when TurboTax shows you the error to enter 0 into Column (b) for periodic withholding.
It is H&R Block for me next year. Erroneous calculations and tax forms = lost business for TurboTax.