How to submit 2020 return after neglecting to press transmit button to finalize. (Current tax year is 2021)

I have a friend who used the CD version of 2020 Turbo Tax Deluxe for the first time last tax season (2020 tax season). In doing his 2021 return he discovered he never pressed the button to file the tax electronically. The questions are:

 

Can we still use the 2020 Turbo Tax CD to finalize his 2020 return? I assume there may be interest and perhaps a penalty?? How will we discover what those might be and how to pay them?

 

Lastly, His 2021 return was rejected with code: IND-031-04. Is it likely that the fact his 2020 tax return was not sent (& therefore not rec'd) is the cause of the 2021 rejection. To be more specific, the code suggests "prior year AGI or self selected pin from last year does not match IRS records". Is his missing 2020 return the reason for 2021 rejection?