I just received an additional 1099S after I filed my 2020 taxes. How do I do an addendum or can I put this on my 2021 taxes without a penalty? [removed]

You cannot claim the 2020 1099S on your 2021 income tax return. It must be reported for the year it was received.  

Use these instructions to file an amended income tax return to account for your 2020 1099S.