If you received a 2021 Form 1099-R that includes code P in box 7 and there is a taxable amount shown in box 2a, that taxable amount goes on the 2020 tax return. If not already reported, you would do that by amending the 2020 tax return. If the amount is small enough that it doesn't require any significant amount of tax due with the amendment, a failure to amend might be ignored by the IRS.
