@ken-golkin
If your box 2a has a non-zero number in it..(not blank, nor marked as undetermined), and that number is somewhat lower than the box 1 number, due to an after-tax contribution amount that is noted in box 5...then when you get to the following page (shown below), answer it as-if the displayed edits were present (i.e. you answer YES).
Answering YES will use the box 2a $$ amount on your for 1040. Check lines 5a and 5b on your form 1040 before and after entering that 1099-R.
(But if you answered No, and already entered some $$ in the "General" or "Simplified" calculation area ,you might need to delete your 1099-R first and re-enter it from scratch...not sure of that though)
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____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*