I'm being asked to enter a value in box 16 of my 1099-R but the 1099-R does not have that box on it.

...as long as box 2a is not empty and contains a non-zero value...yes then box 2a goes into 16.

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If box 2a is empty or marked as "undetermined" in 2b, then it's the Federally-taxable portion of box 1.  Sometimes that is just box 1, but can be lower if there were non-deductible (after-tax) contributions to the retirement plan that issued the 1099-R.

Thanks SteamTrain! That’s what I thought, and you confirmed it. Forging ahead with my tax prep!