A W-2 dated 2018 would need to be entered on a 2018 tax return and not a 2019 tax return. If you filed a 2018 return that did not include the W-2, you would need to amend the 2018 return.
See this article for assistance: https://ttlc.intuit.com/community/amending/help/how-to-amend-change-or-correct-a-return-you-already-...
