Creating Trigger For Views

I have a table tblcustomer and a view called vworder. I need to create a trigger such that any data being added into vworder first checks if the field customerid has the data in customerid of tblcustomer.. all it has to do is spit out a error "Customer ID not found"

So I created a view such as

Quote: create or replace TRIGGER trig_order
BEFORE INSERT OR UPDATE ON vworder
FOR EACH ROW
DECLARE
cust_id VARCHAR2(20);
BEGIN
SELECT customerid INTO cust_id FROM tblcustomer WHERE customerid = :new.customerid;
EXCEPTION
WHEN NO_DATA_FOUND THEN
Raise_application_error(-20000, 'Customer ID Does Not Exist');
END;

But it comes with error

Quote: ORA-25001: cannot create this trigger type on this type of view 25001. 00000 - "cannot create this trigger type on views" *Cause: Only INSTEAD OF triggers can be created on a view. *Action: Change the trigger type to INSTEAD OF.

I am not an expert at all and I need to finish this today itself.