Box 16 is empty or absent on most 1099-R forms (due to various situations)
But the state distribution $$ amount is still usually required for many state forms...and is usually entered by you as the value of box 2a (or the Federally-taxable amount of box 1 if box 2a is empty or "undetermined").
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*