There are 4 reason for this issue.
- The curl/fopen is not enabled in the server
- The firewall is restricting the connection to facebook.
- The entered fb app details are not correct. (Please reset your app secret password and try again)
- The app is not approved with all permissions.
Please contact your hosting team to check the first 2 points (curl/fopen and firewall).
For the third issue, please make sure that your have created the app using the instructions in How can I create Facebook application?