I am trying to fill out my form and it wont let me put the state of Tennessee in box 15 of my W2 and it is the only state missing.

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