Facebook does show the “API Error Code: 191″ when you didn’t enter a matching URL to the Website Tab within the Facebook App Settings. The complete error message is as following:
API Error Code: 191
API Error Description: The specified URL is not owned by the application
Error Message: redirect_uri is not owned by the application.
Please go back to your Facebook App Settings page and make sure that the URL you entered within the Website Tab does match the one from your domain. As Example: if your domain is http://www.yourdomain.com then you need to enter the exact same one on to the Facebook App Settings. if your domain d is http://yourdomian.com than you need to make sure, that you also don’t use the www within the Facebook App Settings.
After you changed that, it can take some time to get Facebook recognize the new settings. Please wait several minutes before you test again.