My husband's Illinois state tax return says line 21 Military pay should be blank but he is active duty army so he's been paid by the military what is the problem?

First go back to "My Info" and edit your military spouse's data to verify IL has been selected as the resident state and he didn't live in any other state.  The same should be true for you (assuming you wish to claim the same resident state as your military spouse to take advantage of the Military Spouse Relief Act).  Here is a link to the IL tax publication for military families.  

Next go to the "Income & Expenses" tab and edit the military W2 to be sure the follow on questions have indicated "Active Duty Military Pay."

Finally, go back and edit the IL return and be sure full year resident for IL is selected and the military pay is excluded.  It should self-populate once you have completed the steps above.