Select the "Settings" panel and specify a valid "Contact email"
---> Save changes
After that select the "Status & Review" panel and move the slide near your app title on YES.
(see also here )
(I suppose you have already created an App adding a platform of type "website" and you have correctly specified the "app domain" and the "site url" - see your q2a "Site URL").
Verify also that you're using the correct "App Id" and "App Secret", and not those of another app :-)
It should work