We have additional income, which I can't get to. The 2020 return is identical to the 2019 return. Let's use it. Is this clear?

No---not clear.   What are you trying to do?   The online software program is only for 2020 returns and you may only enter 2020 income.   Make sure you are using income documents that say 2020 on them---not 2019.

I think what I ought to do is start over, wipe out everything, and do it again. Just so you know, my wife and I each have two retirement income sources, which  I havn't been able to add into the 2020 return.

did you use TurboTax for 2019?  if yes, are you in the same account that was used for 2019.  if yes, did you have the retirement income in 2019?  if yes, the 1099-R's should be proforma'd for the 2020 return so all you need do is enter amounts and codes and possibly update addresses.