Could still be the box 16 distribution amount. If you have withholding in box 14 of that 1099-R, some states complain similarly when box 16 is empty. On either the main form page for box 16, or on a page following the main form, you should be able to enter the value of box 2a into the box, as the Iowa state distribution amount.
That amount has to be greater than box 14. If box 2a is empty or Undetermined use the box 1 value, but you can't have had more IA taxes withheld, then the gross taxable amount distributed.
____________*Answers are correct to the best of my knowledge when posted, but should not be considered to be legal or official tax advice.*
