Per the Iowa Instructions, page 4:

If you qualify for the low income exemption, enter the words “low income exemption” in the area to the left of your net income figure on line 26. Enter zero on line 56 and complete the remainder of the return.

This is the reason line 50 and other lines may not calculate. When the IA return is printed, you will see "low income exemption" next to line 26, and lines 27 through 57 will be blank or zero.