see requirements in installationdocumentation: Safe Mode must be turned OFF
It's a setting in PHP.ini (see PHP info- button in your administrationscreen (bottomleft)) If you installed Moodle on your own server, you can change it yourself, if you work on a host, The best thing you can do is ask your host to turn it of or move to another host.
Enter "safe mode" in the searchfield of using moodle and you'll find a lot of information (but the summary I put up here) on this subject.
Good luck