error ORA-02298 cannot validate - Parent keys not found

alter table t1  enable constraint FK_T1;

 ALTER TABLE emp1 ADD CONSTRAINT fk_orders1 FOREIGN KEY (deptno) REFERENCES dept(deptno);
select deptno from dept 
    DEPTNO
----------
        10
        20
        30
        40
Insert into EMP1  (EMPNO, ENAME, JOB, MGR, SAL, DEPTNO, HIREDATE, COMM)
 Values  (123, 'SMITH', 'CLERK', 7902, 153, 50, TO_DATE('06/23/2011 16:21:41', 'MM/DD/YYYY HH24:MI:SS'), 8);
ORA-02291: integrity constraint (ACXIOM_RD.FK_ORDERS1) violated - parent key not found