Recently, while trying to add one of my Android app into Facebook Audience Network I got this message “This app is already being monetized: The app you’re attempting to add is already being monetized by a business.”. This is weird as I managed to add the iOS version successfully prior. I then tried to remove the app from https://business.facebook.com/settings/monetization-properties and add it again but it still did not work. I later stumble upon a post that mentioned to change the Property name (Integration -> Properties -> Edit Property), I changed mine to a different name and it work! Not sure why it works but hey at least now I can proceed. Hope this helps someone else that is facing the same issue.
Dream Pig Games 