I think you picked the wrong 1099-R to report. It sounds like you have a federal retirement reported on a 1099-R and to report this type of retirement, you need to pick that this is a CSA-1099R.This form does have a Box 12 to report your state tax withheld.
Delete the 1099-R you are working on and then reenter as a CSA-1099-R.
(Be sure you are entering a "2020" 1099-R, and not a CSA- or CSF-1099-R form, from OPM )
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1) The box numbers were changed for a standard 2020 1099-R, and TTX uses those changed box numbers that the IRS wants . Last year's (2019) boxes 12,13,14 were changed to 14,15,16 this year (2020)
2) A few issuers have screwed up and FAILED to update theri forms for the 'proper" 202 box numbering.
3) SO...you look at the "subject" of each line on your paper form, and copy it's value to the same "subject' box on the TTX software form.
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CSA- and CSF-1099-R forms are different, and if you have one of those....you're probably entering it in the wrong TTX software form.