I believe you can safely change the new hire's withholding fields back to the 2020 settings. The clue is that the error referenced FIT 19. The number at the end represents the tax year for the formula. I've heard from others who ran into the same problem. Here's what probably happened. Your current period was probably still set to December when you ran the payroll. That isn't a big problem but it will make the check date default to 12/1/2019 instead of 1/xx/2020. When you put in the ID for the new employee Sage 50 will try to find the appropriate tax formulas for that employee based on the check date. The problem is that 2019 formulas (FIT 19) don't exist for the new W4 choices resulting in an error.

If you simply change the check date to anything in 2020 you will avoid the error message.