Shop getting error from FB and the error is issue is mainly due to the user who is accessing/signing in to Facebook does not have access in development mode, but they tested and customers are also getting the error. Replicated by support.

Full error: "App not active: This app is not accessible right now and the app developer is aware of the issue. You will be able to log in when the app is reactivated."

Also got this error while trying to login as customer on app via FB: "The domain of this URL isn't included in the app's domains. To be able to load this URL, add all domains and subdomains of your app to the App Domains field in your app settings."