I have a 1099-R with an amount in Box 12 (State tax withheld). In TurboTax Deluxe 2020 State tax info is in Boxes 14-16. Should I use Box 14 for State tax withheld?

Make SURE it's just a plain 1099-R form, and not a CSA- or CSF-1099-R, 

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..............several retirement systems/issuers have released 1099-R forms with improperly labelled boxes at #'s 12 and higher for 2020.

For 2020, The IRS changed the box # labelling for boxes 12 and higher.   The former 12,13,14 are now supposed to be boxes 14,15,16 respectively.   So either use that info to put your data in the proper boxes....or just carefully match the subject text on each box before transferring the data it contains to the TTX form.

Then call the issuer, and ask them if they are aware of the issue, and if they are going to correct their forms or not...they do need to get lots of complaints, so they get on the ball and  get it right next year.

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But if your paper form is a CSA- or CSF-1099-R form, then you are entering that form in the wrong form-type in the software.   CSA- and CSF-1099-R forms use a slightly different format and box numbering from a plain 1099-R.

Thanks for your help.  It is a 1099-R.  I will contact the issuer for all the good it will do.