Box 20 only needs to be entered if there is an amount in box 19. If there is no amount in box 19 on your W-2, clear out all entries in boxes 18-20 and leave them all blank (do not enter 0). You will be able to e-file if these are all blank. In the 'Associated state' box, leave it as 'Select a State'.
If there is an amount in box 19, you will need to find out from your employer what to put in box 20.
