Your best option is to enter 01/01/2020 as her date of death on the 2020 return. This will allow you to complete the return, then print and file by mail. You can "white-out" the date of death in the printed return and enter the correct date. This approach also avoids amending the 2019 return (see below).
If your mother passed away in 2019, her 2019 return would have been a "final return" - the IRS cannot accept an e-filed return after the taxpayer's SSN has been identified as belonging to a deceased person.
(The IRS still may not be able to process this return in a normal manner for the same reason it won't allow you to e-file - the SSN is listed as belonging to a deceased person. You will, however, have reported the waiver of debt.)
Your only other alternative is to report the "waiver of debt" on an amended 2019 return. You still won't be able to e-file, but the "waiver of debt" will be reported on a return that the IRS may be able to process using its normal procedures.
See How to Amend (Change or Correct) a Return You Already Filed for instructions on amending
