You must check the "Served in the US armed forces in 2020" box at Personal info, in the federal section, to get TurboTax to give your the right questions in the rest of the interview.
After you enter the W-2 there's a screen with a bunch of check boxes asking if any of those situations apply. One of them is Active Duty Military. If that box is checked, then the Ohio state program will ask about the deduction for military stationed outside of Ohio. It should be the first topic listed on the "Here's the income that Ohio handles differently" screen.
With a valid DFAS EIN, on your W-2, the Military W-2 box should automatically be checked.
In the Ohio interview, it will first ask you if you were in the military, then it will explain what qualifies for the deduction, then it will ask you to enter the amount.
