What is the crowd's opinion of AppZapper?

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In theory, these are the only things you should really have to do:

• Have a local and remote backup (I personally use Time Machine and Backblaze).
• Install updates as soon as they're available (within reason).

Keep in mind the Mac does routine background maintenance tasks, so you shouldn't need to do a lot of this yourself.

Here are some additional things you can do:

• Clean up your Downloads folder every once in a while.
• Restart your computer and web browser(s) at least once a week (each day is probably ideal for performance).
• Calibrate your battery.
• Have an app that cleanly removes Applications when you trash them (AppCleaner, AppZapper, are a couple)

This article is a little old, but still mostly relevant (from my experience).

AppZapper is a great way to delete apps and all their associated files. It removes files that would otherwise stay on your Mac if you only delete the app. There is also an option to keep the additional files if you want to keep them for some reason. https://appzapper.com

Are you trying to run it from /Applications or ~/Applications? Could be a problem there. I'd also suggest using something like AppCleaner or AppZapper to first try to remove all vestiges of previous installs before reinstalling.