Per the IA Net Income Expanded Instructions webpage:

Qualifications for Exemption from Tax

If you qualify for the low income exemption as explained below, enter the words “low income exemption” in the area to the left of your net income figure on line 26. Enter zero on line 55 and complete the remainder of the return.

This is the reason Line 50 and other lines may not calculate in the TaxAct program. 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.

Note that any link in the information above is updated each year automatically and will take you to the most recent version of the webpage or document at the time it is accessed.