Reading round there seems to be several possible causes:
– security plugins can “harden” WordPress by disallowing the editor, check the plugin settings
– some hosts have a “manage wordpress” account and these limit the editing capabilities
– check wp-config.php in the root of your site, and make sure you don’t have something like define(‘DISALLOW_FILE_EDIT’, true);
– networked blogs: you have to click on the “Network Admin” link in the top right-hand corner, and then a Themes section appears in the left column. In that section, there is an Editor option.
– WordPress did not fully install, try reinstalling it
– disable all plugins to see if one of them is causing the issue
– try the default TwentyFourteen theme to be able to rule out theme issues
– if that list doesn’t resolve it, please post your url. There may be other tests that can be done.
