SQL> create or replace trigger trg_biu_emp
  2    before insert or update
  3    on emp
  4    for each row
  5  declare
  6    exc_no_mgr exception;
  7  begin
  8    if :new.mgr is null then
  9       raise exc_no_mgr;
 10    end if;
 11
 12  exception
 13    when exc_no_mgr then
 14      raise_application_error(-20001, 'You have to specify employee''s manager');
 15  end;
 16  /

Trigger created.

SQL> insert into emp (empno, ename) values (99, 'Littlefoot');
insert into emp (empno, ename) values (99, 'Littlefoot')
            *
ERROR at line 1:
ORA-20001: You have to specify employee's manager
ORA-06512: at "SCOTT.TRG_BIU_EMP", line 10
ORA-04088: error during execution of trigger 'SCOTT.TRG_BIU_EMP'


SQL> select empno, ename from emp where mgr is null;

     EMPNO ENAME
---------- ----------
      7839 KING

SQL> update emp set comm = 1000 where ename = 'KING';
update emp set comm = 1000 where ename = 'KING'
       *
ERROR at line 1:
ORA-20001: You have to specify employee's manager
ORA-06512: at "SCOTT.TRG_BIU_EMP", line 10
ORA-04088: error during execution of trigger 'SCOTT.TRG_BIU_EMP'


SQL>