Delete the 1099-R entry and reenter. Also check to see if you may have entered a CSA-1099R by mistake, which does have a Box 12.
[Edited 02-18-2021|02:33 PM PST]
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I've seen that "12a" error being posted for NY over and over and over again.
I "think" there is an error page for NY residents, that has not updated the text in the error notice from prior year's box 12 to the new 2020 format as being box 14 . (but yeah...they do have to make sure it's not a CSA- or CSF-1099-R)