Debugging Oracle DDL triggers

Question

Can't understand why Oracle did not fire any DDL trigger.

>SELECT * FROM V$VERSION 
Oracle Database 18c Express Edition Release 18.0.0.0.0 - Production

Example from https://docs.oracle.com/en/database/oracle/oracle-database/21/lnpls/plsql-triggers.html#GUID-6CF3A208-0BE7-45FF-928C-A755526933D0

STUDENT> CREATE OR REPLACE TRIGGER drop_trigger
           BEFORE DROP ON student.SCHEMA
           BEGIN
             RAISE_APPLICATION_ERROR (
               num => -20000,
               msg => 'Cannot drop object');
           END;
[2022-04-29 11:28:09] completed in 293 ms
STUDENT> create table  tbl1 (a integer)
[2022-04-29 11:28:16] completed in 124 ms
STUDENT> drop table tbl1
[2022-04-29 11:28:17] completed in 155 ms

user and schema - both "Student";

Absolutely the same with DDL-logging example from http://www.dba-oracle.com/sf_ora_04098_trigger_string_string_is_invalid_and_failed_re_validation.htm - no any results for

after ddl on schema 
after ddl on student.schema 
after ddl on database   -- created from name of SYS user

All triggers a visible and shown as ENABLED at all_triggers . There no problem-related message at user_errors or all_errors

Any suggestion?