Help requested to remove apex developer users; making sense of the debug msg.

Hello. I've inherited a problem from the prior DBAs. Having the apex_application_owner role, I should be able to execute apex_util.drop_user(). Instead, I raise an error that appears to say I don't have sufficient privilege(s). Details and debug in the attachments.

OTOH I can use apex, connect as internal, find a user, and delete it/them using the SQL Worksheet wizard - for less than a day, that is. The following day I discover that something has somehow recreated the user account. There are scheduled maintenance jobs but nothing to suggest that there's a script for this.