Help with triggers

I need help urgently with this trigger

Following is the table on which I am trying to run the trigger:

create table bookings
(
Booking_no NUMBER primary key,
Cust_no NUMBER,
Date_reserved DATE,
Reserved_by CHAR(12),
Date_rent_start DATE,
Rental_period NUMBER,
Registration CHAR(7),
Model_name CHAR(8),
Miles_out NUMBER,
Miles_in NUMBER,
Amount_due NUMBER,
Paid CHAR(1)
);



I am trying to create a trigger that would print cust_no when the sysdate-date_rent<=3


here's the trigger:

create or replace trigger ALREADY_BOOKED
before insert on bookings
FOR EACH ROW
DECLARE
BOOKED_NO NUMBER;
CUSTOMER_NO NUMBER;
BEGIN
CUSTOMER_NO := :new.cust_no;
select COUNT(*) INTO BOOKED_NO from bookings where (sysdate-Date_rent_start) <=3 AND bookings.Cust_no = :new.Cust_no;
IF BOOKED_NO >0 THEN
DBMS_OUTPUT.PUT_LINE(:new.cust_no);
END IF;
END;
/

The trigger doesn't work.

Thanks in advance.