Going to need a bit more info. Hide All Applications works for me in a simple test macro. This is on the latest version of Big Sur with Keyboard Maestro 9.2.
What exactly is "not working"? For example do you mean some apps do hide, but others don't? or that nothing at all happens.
If some apps hide, but others don't, do you have any apps in your Excluded applications list? (Keyboard Maestro Preferences > Excluded tab). You can turn on the "Hide applications on the excluded applications list" if this is the case and you want to hide those apps. The excluded list is empty by default and since you're just starting out I doubt this is the case but might as well double-check.
If it's the "nothing at all happens" I'd make sure your macro is actually triggering. Add a pop-up window to the beginning of the macro to clearly indicate it at least got that far.
Thanks for the reply. I have nothing in the Excluded tab in Preferences, and don't have Hide applications on the excluded applications list checked.
By "not working" I mean that no apps are hidden at all when I either use the hotkey or the Try Action option. If I switch the action for Hide a Specific Application or Hide Other Applications, they work as described, but not Hide All Applications.
I've ruled out hotkey conflicts by swapping the Hot Key Trigger with Time of Day and Typed String triggers. It's definitely the action rather than the trigger, and specifically the Hide All action rather than any other action. I've worked through a number of KM tutorials last week, and haven't encountered another unresponsive action.
See OP bullet #3. I've tried disabling and reenabling KM and the KME in the accessibility settings during and prior to running the them, as well as prior to restarting macOS. Maybe I'll tried uninstalling and reinstalling them.
hahaha, actually I use PathFinder too, but I don't have it quit the Finder (tried it years ago and it was really annoying, I think mostly because OS X kept relaunching Finder anyway). So even if I had known you were using it I probably wouldn't have thought about the quit Finder option.