You will have pay taxes on the earnings in the year you made the contribution for. Therefore, the 2021 Form 1099-R with the code P belongs on our 2020 return.
If you had created the Form 1099-R when you filed your 2020 tax return as macuser_22 had suggested then you can ignore the Form 1099-R with code P unless there is Box 4 Federal Tax withholding and/or Box 14 State withholding. Then you must enter the 2021 Form 1099-R with code P into the 2021 tax return since the withholding is reported in the year that the tax was withheld. The 2021 code P will not do anything to the 2021 tax return but the withholdings will be applied to 2021.
If you didn't create Form 1099-R with code P on your 2020 tax return then you will have to amend your 2020 tax return to report the Form 1099-R with code P.
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