The contribution for 2020 was reportable on your 2020 tax return, either as deductible on Schedule 1 line 20 or as nondeductible on Form 8606 line 1, not on your 2021 tax return. If the 2020 contribution was nondeductible, the amount from line 14 of your 2020 Form 8606 carries forward to line 2 of your 2021 Form 8606 to be used in the calculation of the taxable amount of your Roth conversion performed in 2021.
If you did not report the 2020 contribution on your 2020 tax return, you need to amend 2020. If the contribution was nondeductible, that amendment will include Form 8606. (Sometimes you can get away with filing the missing Form 8606 stand-alone, but that assumes that the correction causes no other changes to your originally filed tax return.)
