Another user's Windows environment
"This game's Windows environment belongs to …, another user of this Mac": the game is in a Steam library you share with another person on this Mac, and their Superposer made its Windows environment in a place that keeps file ownership (the Shared folder, or an external drive with "Ignore ownership on this volume" off). Windows games only run in an environment you own. Install the game in a library of your own, or turn Ignore ownership on for the drive (Finder › Get Info); then you share that environment, saves included.