My state does not have state income tax, however, the form is not letting me move forward because I'm unable to enter TN into box 15.

Tennessee will not appear as a choice, because Tennessee does not have an income tax. It is ok to leave this blank.  If you are receiving an error, it could have been an issue caused by importing.  If you are unable to delete the information to leave the state blank, you can delete the W-2, and re-enter it manually.

Leave boxes 15, 16 and 17 blank (empty) on the W-2 you are entering.

If you still get an error message then delete the W-2 and re-enter manually.