Auditing procedure ORA-01031

Hi

I would need to create a stored procedure in order to enable day by day audit on db users, I created the following procedure in SYSTEM user:

CREATE OR REPLACE PROCEDURE SYSTEM.ACTIVATE_AUDIT AS
 stmt_audit varchar2(100);
BEGIN

 for c1 in (select d.username from user_view.v_dba_users d where d.account_status='OPEN' and substr(d.profile,1,4) not in ( 'TECH','ADMI','HIDS','ESM_') )
 LOOP
  stmt_audit := 'audit all by ' || c1.username || ' by access whenever successful';
  execute immediate stmt_audit;
 END LOOP;
END ACTIVATE_AUDIT;
/

but when I run it gives

ORA-01031: insufficient privileges