You can get this message if the app is not made available to the general public.
Since the UI of the facebook has changed recently, here’s the latest update for setting the sandbox mode ON/OFF :
Go to https://developers.facebook.com/
Click on the Apps menu on the top bar.
Select the respective app from the drop down.
Go to ‘Status & Review’ from the table in the left side of the page.
Do you want to make this app and all its live features available to the general public? – Select switch to set YES/NO value.
