There are many reasons, check the one which apply to you: 

1) Incorrect copy paste of app ID and/or app secrets ID 

2) You try to activate app from different Facebook account than their account on which you added FB app 

3) On app setting domain should be 

4) You forget to add a web platform to app or the URL entered by it is not

5) Check if you switched the app to live by switching live to YES.