OK I see now, thank you for the screen shot it really helped.
I have googled and it is because I installed iThemes Security Pro after my site has been hacked and it add this code to wp-config.php
// BEGIN iThemes Security – Do not modify or remove this line
// iThemes Security Config Details: 2
define( ‘DISALLOW_FILE_EDIT’, true ); // Disable File Editor – Security > Settings > WordPress Tweaks > File Editor
// END iThemes Security – Do not modify or remove this line
So I have unticked this setting and you can see the editor now, hope it will help.
Best regards,
Mag