create or replace trigger auto_create_dml_trigger
after create on database
declare
pragma autonomous_transaction;
create_sql varchar2(2000);
begin
create_sql := generate_trigger_exp( ); 
--comment: generate_trigger_exp() is a 
--procedure used to generate an expression that creates a trigger. I assure you I can 
--succussfully get the whole expression. For example, i create a table table_a, then
--auto_create_dml_trigger got fired, and create_sql will get the value 'create or replace 
--trigger syn_update after update on table_a ......' by generate_trigger_exp().
execute immediate create_sql;
commit;
end;

So you've got a DDL trigger that runs more DDL. So this will happen:

Creating a table fires the DDL trigger.
Which creates a trigger
Which fires the DDL trigger again
Which tries to create a trigger again
Which fires the DDL trigger AGAIN
Which tries to create a trigger AGAIN!
etc.

See the problem?

Depending on how generate_trigger_exp works, it may be trying to re-create the SAME trigger. Which it's going to get stuck doing!

I'd avoid trying to auto-generate triggers like this. If you need a trigger code it explicitly.

Or look to see if there's an existing feature that does what you're planning in the trigger. e.g. IDENTITY columns/sequence defaults for auto-incrementing columns.