I would like to ask regarding this error...

   Error 49 at line 5, column 6
       bad bind variable 'S_ORD.payment_type'

Here is the code:

    DECLARE
     N NUMBER;
      v_credit S_CUSTOMER.credit_rating%type;
    BEGIN
      IF :S_ORD.payment_type = 'CREDIT' THEN
        SELECT credit_rating
        INTO v_credit
        FROM S_CUSTOMER
      WHERE :S_ORD.customer_id = id;
      IF v_credit NOT IN ('GOOD', 'EXCELLENT') THEN
            :S_ORD.payment_type:= 'CASH';
          n:=SHOW_ALERT('Payment_Type_Alert');
      END IF;
     END IF;
   END;

I'm new to oracle forms so I'm not sure if I have a missing setup or anything. S_ORD table exist and has a column payment_type, which consists of 'CREDIT' and 'CASH' value. Thank you.