On my Form 1099R Box 16 State Distribution is blank but the program wants something in that space.

Simply enter the taxable amount shown in 2a on your 1099 (assuming your state taxes your income).

Generally, many of us recommend putting the box 2a value in there...as long as box 2a has an actual non-zero value in it.

If box 2a is empty, or marked as being "undetermined"....then as long as you didn't put make an "After-tax" contributions to that type of retirement account, then you could likely just enter the box 1 value into box 16.