The 2020 non deductible contribution for 2020 should have been reported on the 2020 return creating a form 8606 that will in turn be used on the 2021 return. In the 2021 return when asked if you had prior year non deductible basis from line 2 of the 2020 8606 you would enter the $6000 to be added to the 2021 non deductible contribution. So go back to that section and make the correct entry. If you did not complete an 8606 on the 2020 return you need to amend that return to add it.
