I would also assume that there is no state tax wages in box 16 or withholding in box 17 because Washington does not have income taxes. Therefore, do not enter WA in box 15 either or the software will prompt you for the additional information, including the state ID number.
You can see under the state taxes tab that there's no state return available for Washington because they don't have an individual income tax return filing requirement.
You can read more on the state of Washington's government website HERE.
