Facebook login error "User is not allowed to see the application."
Another reason this will happen is if you are logged in as a test user from a different app than the one you are trying to sign in to. Facebook test users are app-specific.
Sign out of Facebook and back in with a test user from the correct app.
Go to Safari > http://m.facebook.com > logout In cases where app sign in with safariviewcontroller, sometimes it contains test user of different app
In my case, I was using wrong facebook application id in the code. So added the correct id & it worked for me.