Have seen that error in the software before...they are really complaining about box 14 (the "a" just refers to the first line for the state entries).
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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.
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IF numbered properly, box 16 is commonly empty on most 1099-R forms. Usually because the value of box 2a is automatically used in it's place (or the Federally-taxable amount of box 1 if box 2a is empty).
NY is especially picky about this, and entering the value of box 2a into box 16 should fix that (or the federally-taxable amount of box 1 if box 2a is empty). That should fix your supposed "error"
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So what if I have values in Boxes 1, 2a and 14 (which is state tax withheld), and there are no values in Box 16. Box 12 is a check box for FATCA filing requirement. Would I input the value from box 1 or 2a into box 16, despite it being greater than Box 14? This is with my TD Ameritrade 1099-R.
Box 16 is left empty on 95% (ok...that's a guess) of 1099-R paper forms that are issued. But the software needs a value in box 16 for many-to-most states that have an income tax. So putting 2a (as long as 2a has a real value in it) into box 16 avoids issues with that. Generally, the software takes care of doing that as you continue in the interview past the main1099-R page, where there is one that displays the state "Distribution" amount....which is actually what the software is putting in box 16. For the software, Box 16 must be greater than 14....you can't have tax withholding (14) taken out of nothing.