If you did not pay any mortgage interest in 2021, you don't enter anything. You don't have to tell the IRS that you paid off the mortgage. You just don't enter anything about a mortgage.
If you paid off the mortgage in 2021, and paid some mortgage interest before you paid it off, you should get a Form 1098 from the lender. You just enter the interest that you paid, as shown on the 1098. You do not have to tell the IRS that you paid off the mortgage.