Trigger with compilation errors

Well it turns out that I want to make a trigger that when I update a column in a specific table records are annexed ie I have a table where articles when the sale is made and the quantity is discounted that information is given to a sales table. acontinuacion show the code:

create table articulos( 
id_articulo number(5) primary key,
nombre varchar2(32),
detalle varchar2(32),
precio number(8) not null,
cantidad number(10) not null,
);
create table ventas(
    id_venta number(5) primary key,
    id_articulo number(5) references articulos,
    id_empleado number(6) references empleado, 
    id_cliente  number(5) references cliente, 
    cantidad number(10) not null,
    precio number(8) not null,
    total number(8) not null,
    fecha_venta date
); 

The trigger is as follows:

 create sequence id_venta
start with 1
increment by 1
order;

create or replace trigger anexventa
before update on articulos
referencing new as new old as old
for each row
declare
x number;
begin
select id_venta.nextval into x from dual;
 :new.id_venta := x;
if updating('cantidad') then
insert into  ventas values(:NEW.id_venta, :old.id_articulo, :old.id_empleado, :old.id_cliente, :NEW.cantidad, :NEW.precio, :NEW.total, sysdate);
end if;
end anexventa;
/

As you can see, attach a sequence which allows me to generate the sale id automatically.

The errors thrown at me are:

SQL> show errors trigger anexventa;
Errors for TRIGGER ANEXVENTA:

LINE/COL ERROR
-------- -----------------------------------------------------------------
5/2      PLS-00049: bad bind variable 'NEW.ID_VENTA'
7/28     PLS-00049: bad bind variable 'NEW.ID_VENTA'
7/61     PLS-00049: bad bind variable 'OLD.ID_EMPLEADO'
7/79     PLS-00049: bad bind variable 'OLD.ID_CLIENTE'
7/124    PLS-00049: bad bind variable 'NEW.TOTAL'

I hope someone can guide me regarding the problem. Greetings.