NO NO NO
You cannot put a 2019 W-2 into your 2020 tax return. If you left it out of your 2019 return then you need to amend your 2019 return. You absolutely cannot include it with 2020 income.
If your 2019 return has been fully processed, then amend your 2019 return.
https://ttlc.intuit.com/community/amending/help/how-to-amend-change-or-correct-a-return-you-already-...
DO I NEED TO AMEND/
https://ttlc.intuit.com/questions/1908543-do-i-need-to-amend
https://www.irs.gov/filing/individuals/amended-returns-form-1040-x/amended-return-frequently-asked-q...
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
