Why won't Move back work when a copy of the app is already on my Mac?
Short answer: A real copy of the app already exists in its original folder, and Storage Studio won't overwrite it. Quit the app, move that copy to the Trash, then try Move back again.
Storage Studio never overwrites an app it didn't put there. If a real copy of the app already sits in the original folder — often because an updater reinstalled it — moving back would replace it, so Storage Studio stops and tells you instead.
If an update caused it, the reinstall watch usually tidies it up by itself once you quit the app. Otherwise, quit the app, drag the copy in the original folder to the Trash (it stays recoverable there), and try Move back again.