Why is TN not listed to be entered where the turbotax system inquires about box 15 which prohibits completion?

TN is not in the list for box 15 because Tennessee has no income tax on wages. Boxes 15 through 20 should not be used on a Tennessee W-2. If box 17 is blank or zero, leave boxes 15, 16, and 17 empty. If there is nothing in box 17, it's not necessary to enter TN in box 15, even if it's on your W-2.

Tennessee does not have a state income tax, so no tax should have been withheld. 

Is there an amount in Box 16 or Box 17 for the state? If Box 16 and Box 17 are blank, you can leave Box 15 blank. 

@tdking14