Not sure if this is the right place since it is not a question.
So I had this problem and had studied every page I could on this and it didn't work. Checked the settings, turned off sandbox mode and even created a new app. Nothing helped.
But I was able to get it to work just exploring. So in application.facebook.com, I went into the App. I then added platform for website. I turned off Sandbox. The last step:
I went to App Details
Scroll about halfway down and find App Center Listed Platforms
For me, Website was set to Off. I turned it on