Hello - on IL tax return I am asked to enter 'Military pay Exclusion - Qualifying Military Pay'. On my W2 form what is the box number should I refer to?

Box 1 of your DFAS W-2.

Tax exempt military pay will be in box 12 with either a code J, code P or code Q.

Illinois does not tax military pay earned by service persons.  OP was specifically asking about Illinois.

Negative. I have D 21 and AA code in Box 12