You do not enter anything in your 2021 tax return about the federal tax you paid for your 2020 tax due. It is not deductible, and there is no place to enter anything on a new federal tax return about how much you paid the previous year. If you still owe on the payment plan, then any refund you could get for 2021 will be offset by the amount you still owe to the IRS. The IRS knows how much you owe, and will do the calculations.
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