ORA-02291 indicates the database couldn't find a matching parent key value

CREATE TABLE departments
(department_id             number(10)            not null,
 department_name           varchar2(50)      not null,
 CONSTRAINT departments_pk PRIMARY KEY (department_id)
);

insert into departments ( department_id, department_name )values(1,'Data Group' );
insert into departments ( department_id, department_name )values(2,'Purchasing' );
insert into departments ( department_id, department_name )values(3,'Call Center' );
insert into departments ( department_id, department_name )values(4,'Communication' );


CREATE TABLE employees
( employee_id          number(10)      not null,
  last_name            varchar2(50)      not null,
  job_id               varchar2(30),
  department_id        number(10),
  salary               number(6),
  manager_id           number(6),
  CONSTRAINT           employees_pk PRIMARY KEY (employee_id),
  CONSTRAINT           fk_departments FOREIGN KEY (department_id) 
                       REFERENCES departments(department_id)
);


SQL> insert into employees( employee_id, last_name, job_id, salary,department_id ,manager_id)
  2  values(                1001,        'Lawson',  'MGR',  30000, 6,             1004);
insert into employees( employee_id, last_name, job_id, salary,department_id ,manager_id)
*
ERROR at line 1:
ORA-02291: integrity constraint (SYS.FK_DEPARTMENTS) violated - parent key not
found


SQL>