I filed a 2020 W-2 on my 2021 tax returns. I wasn’t aware you could not do this. Now the IRS is holding my return. What do I do?

[fanfare]

"the IRS is holding my return. "

 how do you know that? did you get a letter?

Use Form 1040-X (amendment) to add or correct information on an already filed tax return.

Delete any 2020 W-2 documents.

You have until Oct 15 to e-File 1040-X if you did an e-File of Form 1040.

[SweetieJean]

Did you file that same W-2 on your 2020 return?

[Critter-3]

Do NOTHING until the IRS finishes processing the return and you have a refund in hand or a balance due letter.  Until you know if the IRS caught the error you do not know if you need to amend the  return.  If you have not yet filed the  return (or amend it to add the extra income)  then you can do so now.... you don't have to wait on the 2021 return to process.