There seem to have been a large number of 1099-R forms this year with box 16 blank (or maybe TurboTax is just catching them more).
If you have just one state on your return, the amount that should be entered in box 16 is almost certainly the same as the amount in box 1. So just borrow the box 1 amount.
However, if you do not have any amount in box 14, then don't enter anything for boxes 15 and 16, either.
Some software that creates 1099-R forms automatically enters a state in box 15, even when there is no withholding for box 14.
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