Oracle 18c

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
CREATE TABLE employee(id_emp INT, id_office INT)
CREATE OR REPLACE TRIGGER trg_chk_unique_emp_dept_id
AFTER INSERT ON employee
DECLARE
  val INT;
BEGIN
    SELECT NVL(MAX(COUNT(*)),0)
      INTO val
      FROM employee
     GROUP BY id_emp, id_office;
  IF val > 1 THEN
    RAISE_APPLICATION_ERROR(-20304,
                            'Each employee may be assigned to a department once at most !');
  END IF;
END;
/
INSERT INTO employee 
SELECT 1, 1 FROM dual UNION ALL SELECT 1, 1 FROM dual
ORA-20304: Each employee may be assigned to a department once at most !
ORA-06512: at "FIDDLE_GDIOVRLFAXMPJLBMNMAO.TRG_CHK_UNIQUE_EMP_DEPT_ID", line 9
ORA-04088: error during execution of trigger 'FIDDLE_GDIOVRLFAXMPJLBMNMAO.TRG_CHK_UNIQUE_EMP_DEPT_ID'

INSERT INTO employee VALUES(1,2)
1 rows affected

INSERT INTO employee VALUES(1,2)
ORA-20304: Each employee may be assigned to a department once at most !
ORA-06512: at "FIDDLE_GDIOVRLFAXMPJLBMNMAO.TRG_CHK_UNIQUE_EMP_DEPT_ID", line 9
ORA-04088: error during execution of trigger 'FIDDLE_GDIOVRLFAXMPJLBMNMAO.TRG_CHK_UNIQUE_EMP_DEPT_ID'

INSERT INTO employee VALUES(1,3)
1 rows affected