VARIABLE max_dept_no NUMBER
DECLARE
  v_dept_name VARCHAR2(30) := '&p_dept_name';
  v_max       NUMBER(4,0);
BEGIN
  SELECT MAX(department_id) INTO v_max FROM departments;
  :max_dept_no := v_max + 20;
  INSERT  INTO departments  VALUES    (:max_dept_no,v_dept_name, NULL,NULL) ;
END;
/

describe "HR"."DEPARTMENTS"

Name            Null     Type         
--------------- -------- ------------ 
DEPARTMENT_ID   NOT NULL NUMBER(4)    
DEPARTMENT_NAME NOT NULL VARCHAR2(30) 
MANAGER_ID               NUMBER(6)    
LOCATION_ID              NUMBER(4)    

orcl> VARIABLE max_dept_no NUMBER
orcl> DECLARE
  2    v_dept_name VARCHAR2(30) := '&p_dept_name';
  3    v_max       NUMBER(4,0);
  4  BEGIN
  5    SELECT MAX(department_id) INTO v_max FROM departments;
  6    :max_dept_no := v_max + 20;
  7    INSERT  INTO departments  VALUES    (:max_dept_no,v_dept_name, NULL,NULL) ;
  8  END;
  9  /
Enter value for p_dept_name: new
old   2:   v_dept_name VARCHAR2(30) := '&p_dept_name';
new   2:   v_dept_name VARCHAR2(30) := 'new';

PL/SQL procedure successfully completed.

orcl>

Error starting at line 2 in command:
DECLARE
  v_dept_name VARCHAR2(30) := '&p_dept_name';
  v_max       NUMBER(4,0);
BEGIN
  SELECT MAX(department_id) INTO v_max FROM departments;
  :max_dept_no := v_max + 20;
  INSERT  INTO departments  VALUES    (:max_dept_no,v_dept_name, NULL,NULL) ;
END;
Error report:
ORA-01400: cannot insert NULL into ("HR"."DEPARTMENTS"."DEPARTMENT_ID")
ORA-06512: at line 7
01400. 00000 -  "cannot insert NULL into (%s)"
*Cause:    
*Action: