A 2021 W-2 "must" be reported on a 2021 tax return. You can not combine any income from multiple tax years, on a single tax year tax return. If you have a 2021 W-2 that you did not report on your 2021 tax return, then you will need to complete and file a 2021 tax return. If you did file a 2021 tax return and it was accepted and processed by the IRS, but just forgot to include that 2021 W-2, then you will need to amend your 2021 tax return to include it.
