On a 1099-R form?
Just enter the box 2a $$ amount in box 16....that is IF..if box 2a has a non-zero number in it.
Otherwise copy box 1 into box 16, as long as you had no "after tax" contributions to that retirement vehicle.
If you did have some After-Tax contributions to it, then you need to put the Federally-taxable portion of box 1 into box 16 (i.e, whatever the Feds included as taxable income)
Essentially the state software is saying that you can't have state taxes withheld from nothing.
Yes, many providers of 1099-R forms always leave box 16 empty, apparently for a few reasons....but some states require a number in there for the software to work correctly