If you have state taxes withheld in box 14 of your Form 1099-R then enter the amount from box1 in box16 if your box 16 has a 0 entered.
The payer has omitted to fill box 16 of your form 1099-R.
You can fill it with the amount in box 2a. If box 2a is blank or 0, then fill box 16 with the amount in box 1.