I think I figured out the problem. You have to enter your YTD withholding earlier in the W4 calculator on the Enter Your 2021 W2 Wage Information. Enter your wages and then YTD withholding. This then populates the Federal Income Tax Withheld fields that are several screens later on the Enter Payments You've Already Made screen. Alternatively, you can pull up the actual form by clicking Show ES & W4 Wks all the way at the bottom of the screen. YTD federal income tax withholding can be entered in the W2 box.
Now, the question is, why doesn't TurboTax allow you to enter / modify federal income tax withheld on the Enter Payments You've Already Made screen. It makes sense to be able to edit the tax withheld YTD here because that's the title of the screen. TurboTax Team should clarify the Withholding on the earlier screen Enter Your 2021- W2 Wage Information to say 2021 YTD Withholding. Otherwise, it is confusing because the 2020 column shows the full year withholding, so it makes you think you have to guess your total withholding for 2021, but you just have to enter the YTD withholding there.
