That’s really weird... that’s an error message you’d normally see while trying to build the game, not while trying to run it.
Can you delete all traces of this app and just try redownloading it?
The “kind” after executing the steps in the video should be “unix executable”, not “application” (of the Use Your Outside Voice file under Contents/MacOS; the kind of the OutsideVoice.app file should be “application”), so I’m not sure if something is getting messed up at that step. As a last ditch attempt you might also try renaming the “Use Your Outside Voice” file to something without spaces (which I probably should have done in the first place!)
Edit: You might also try updating xcode- I don't know if it will help, but apparently it's the fix for when this error appears while trying to build the game.