Is there anything in boxes 16&17? If not then leave box 15 blank. Or ask your employer.
You didn't happen to do some work in the Marianas islands or Saipan did you ? (grasping at straws)
if you did not work in a state with an income tax. I would ignore it. on the other hand, if you did, you really need to check with your employer. You may have gotten an incorrect W-2.
