@karlarson
That is an extremely common problem....thousands of times so far this year, because most payers of 1099-R forms don't put anything in box 16.
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To solve this problem:
1) If box 2a of your 1099-R form has a positive non-zero number in it, copy that value into box 16.
OR
2) If box 2a is empty or "Undetermined", you need to enter the Federally-taxable amount of box 1 into box 16