Box 15 of what? Your W-2?
Several states do not have a state income tax. So these states will not show up in certain drop down lists. Leave boxes 15, 16, and 17 blank when you enter a W-2 for these states.
Alaska, Florida, Nevada, South Dakota, Tennessee, Texas, Washington and Wyoming
If you are talking about box 15 on some other form or schedule, you will need to be specific---there are are lot of "boxes" throughout the tax software -- and we cannot see your screen.