Can't edit classified on front end

Hello everybody,

when trying to edit a classified on the front end the page does not load completely …

Here is the message I have in the log file :

PHP Fatal error: Call to undefined function wp_editor() in /sites/testsite.xyz/wordpress/wp-content/plugins/classifieds/ui-front/general/page-update-classified.php on line 94, referer: http://abc.testsite.xyz/classifieds/my-classifieds/

It looks like there is a missing file inclusion but I can not figure which one …