App Not Setup: This app is still in development mode

"App Not Setup: This app is still in development mode, and you don't have access to it. etc etc"

If you're trying to connect your WordPress site to Facebook and you're presented with this error it's usually because the Facebook Account you're signed in with doesn't have access to the App that's being used. Try logging out of Facebook and logging back in with a User that has access to the Facebook App being used.