ORA-30502: system triggers cannot have INSERT, UPDATE, or DELETE as triggering events
Cause: An attempt was made to create a system trigger with INSERT, UPDATE, or DELETE triggering events, but this type of trigger is not supported because a system trigger does not have a base table
Action: Do not attempt to create a system trigger with INSERT, UPDATE, or DELETE triggering events
https://support.oracle.com/
http://www.oracle.com/