If Dad passed in 2020, Mom should still be filing a Joint return for 2020. If Dad's SSN is locked, she may need to file it by mail.
If, however, he passed in 2019, then you cannot just delete his name from their TT account. Rather, you must set up a new one for Mom, and enter just her info in it. The IRS keeps its record by the SSN of the first person on the return, so she might need to file by mail this time as well.
