when backup disk is full
When backup disk is full, I thought Time machine will automatically deleted oldest backup to make room, but it is not doing it. And keep asking to find a different disk. Please advise!!
Mac Pro, macOS 10.15
It does, and probably is, but the bottom line is that if TM says it needs more space, then it needs more space.
The solution is to obtain a bigger TM backup drive. Or, erase the one you have and start a new set. You should probably get a bigger backup drive first though, because you never want to be without a backup for even a short period of time.
One common reason for running out of space is that TM backups created prior to upgrading to Catalina are essentially archived and are no longer updated.