Grant Role/Grant Privilege

While Creating the following trigger at "Scott" User I've got an error as follows:

create or replace trigger data_logon_trigger
after logon
ON DATABASE
begin
execute immediate
'alter session set nls_date_format = ''yyyymmdd'' ';
end;
/

ERROR at line 3:
ORA-01031: insufficient privileges

I've given the following Grants to Scott
Grant Create Trigger to Scott
Grant Alter Any Trigger to Scott

After Above also I've got the same error Later I've given the following:
Grant DBA to Scott
Still I've got the same error, later I've given the following Grant:
Grant Administer Database Trigger to Scott
Then I can able to create the trigger.

By Default "Administer Database Trigger" is a Privelege is part of DBA role
Even If I 've granted that role to Scott, then why it is not able to created that trigger ...........?
Can u please explain problem might be what..?

Thanks in advance..
Giri K.Y.

logon on schema is what you want