When using event trigger on Greenplum 6.x there is no function pg_event_trigger_ddl_commands does not exist.
The function pg_event_trigger_ddl_commands does not exist in Greenplum 6.x because Greenplum 6.x incorporates PostgreSQL versions 8.4 thru version 9.4. PostgreSQL introduces pg_event_trigger_ddl_commands in version 9.5:
https://www.postgresql.org/docs/9.5/release-9-5.html
Greenplum 7.x is based on PostgreSQL version 12 and supports pg_event_trigger_ddl_commands: