Just tried to delete another user, that works fine. So it has to do with this specific user.
General help
[3.9.1] Can't find data record in database table course.
ការពិភាក្សាត្រូវបានបិទ ដូច្នេះអ្នកមិនអាចឆ្លើយតបទៀតទេ។