Custom certificate (plugin)

I go to reset my couse I get the following error:

Invalid get_string() identifier: 'certificateremoved' or component 'certificate'. Perhaps you are missing $string['certificateremoved'] = ''; in mod/certificate/lang/en/certificate.php?

• line 6603 of \lib\moodlelib.php: call to debugging()
• line 7227 of \lib\moodlelib.php: call to core_string_manager->get_string()
• line 161 of \mod\certificate\lib.php: call to get_string()
• line 4956 of \lib\moodlelib.php: call to certificate_reset_userdata()
• line 73 of \course\reset.php: call to reset_course_userdata()

Using moodle 2.3.1