There is an amount in box 12 for state withholding on my 1099R but the form in turbo tax does not show a box 12. Where do you enter the state withholding amount?

either:

1) that is a 2019 form 1099-R you are entering...and you can't enter a 2019 1099-R in the Online software anymore....only 2020 forms.

OR

2)  The GA issuer failed to update their 2020 1099-R forms to the proper format for 2020. 

For 2020, boxes 12,13,14 were changed to  boxes14,15,16.   If your 1099-R is an actual 2020 form, then you should call the issuer and see if they are issuing corrected forms.

OR

3)   You really have a CSA-1099-R and are entering it as a plain 1099-R and have not selected the proper CSA-1099-R  form during the interview...(or perhaps a CSF-1099-R)

You can enter the info in the correct boxes by ignoring the box numbers ... 12-13-14 changed to 14-15-16  ... just be smarter than the paper form.