WordPress theme editor is missing
Open up your wp-config.php file, and search for
define('DISALLOW_FILE_EDIT', true);
Change true to false:
define('DISALLOW_FILE_EDIT', false);
It is probably because of iThemes Security and its settings. So yeah, just go to wp-config and change DISALLOW_FILE_EDIT to false.
define( 'DISALLOW_FILE_EDIT', false );
Or just switch it off here: Dashboard -> Security -> WordPress Tweaks -> Configure Settings
Uncheck: Disable File Editor