W-2s must be part of a return for Only the year they are for - either an original, or an amended return for the same year printed on the W-2. You can Not use a 2017 (or earlier) W-2 on your 2018 return. If you already filed your 2018 return, and want to add a 2018 W-2, then you have to amend 2018 (complete, print, and mail in a Form 1040X with added W-2).
