Prohibition

While attempting to remove desktop files and clean up, I got the spinning beachball of death and could not get out of the freeze. Force quit worked for some but not all programs. Powered down with off button. When I tried to reboot I got a black screen with apple logo and progress bar that was replaced with a circle slashed line "prohibited "I discovered.

I tried the recovery programs up to option command shift R and wound up with the same.

This is an older desktop Mac that had Mojave on it, which I think was the last install I did, although it may have gotten an update to Catalina when I was too sleepy to care which button I pushed.

Had recently run "Clean my Mac" and was backing up a new iPhone . Have hit a dead end.