Form 1099-INT is giving me trouble. The software tells me I must select my state from the list for box 15, but Washington isn't in the list. How do I get past this?

Washington state does not have income tax.  You can type the two letter abbreviation WA in the box and it will correct the issue.

You say 1099-Int.  Do you mean W-2.  If so, leave boxes 15-20 blank. They are not needed for full year WA residents with no income from other states.