Last year I had a similar problem with form 8606. I spent many hours trying to figure out how to enter the data on the 8606. I wound up having to print out the return and white out the entry that Turbo Tax would not let me enter. I then had to go through the rest of the forms and white out everything that was affected by the change I made on the 8606. After that I made copies of the whole thing and mailed it in. You obviously can't file electronically when you do stuff like that.
It seems like Turbo Tax would have this figured out. I have not done my 2020 taxes yet. I hope they have fixed it.
I have heard that if you get Turbo Tax on a disc rather than do your taxes on line that you can make all the entries on your 8606. I hope this is true because that is what I am trying this year.
Good luck!
John H.
