ORA-01733: virtual column not allowed here

Bellow are my Tble, view, trigger and fuction when try to update the table using the view getting

ERROR at line 3:
ORA-01733: virtual column not allowed here



create table SKU_FUTURE_COST (
      IMAGE_RECNBR number(10) not null
     ,SKU varchar2(20) not null                    
    DATE_COST_EFF date -- yymmdd                
  );

create view VS_SKU_FUTURE_COST
   as select
   IMAGE_RECNBR
   ,SKU
   ,DATE_COST date -- yymmdd
   from SKU_FUTURE_COST
/

create or replace trigger SKU_FUTURE_COST_VIR
instead of insert on VS_SKU_FUTURE_COST
begin
   insert into SKU_FUTURE_COST (
         SKU
        ,DATE_COST_EFF
        ,Date_Add_Trigger
      ) values (
         :new.SKU
        ,fn_FmtCblDate_YYMMDD(:new.DATE_COST_EFF)
        ,sysdate
      );
end;
/

DROP FUNCTION fn_FmtCblDate_YYMMDD;

Create Function fn_FmtCblDate_YYMMDD(coboldate char)
   RETURN DATE AS
Begin
 Declare
  cobolholddate char(6);
  Result date;
 Begin
  IF coboldate = ' ' then
    Result := NULL;
  Else
    If coboldate = '000000' then
     Result := NULL;
    Else
    If length(coboldate) = 3 then
         cobolholddate  := '000' || coboldate ;
         Result :=  to_date((to_number(cobolholddate)), 'yymmdd');
    Else    
    If length(coboldate) = 4 then
         cobolholddate  := '00' || coboldate ;
         Result :=  to_date((to_number(cobolholddate)), 'yymmdd');
    Else      
    If length(coboldate) = 5 then
         cobolholddate  := '0' || coboldate ;
         Result :=  to_date((to_number(cobolholddate)), 'yymmdd');
    Else    
         Result :=  to_date((to_number(cobolholddate)), 'yymmdd');
    End if;
    End if;
    End if;
    End if;
 End if;
  RETURN Result;
 End;
End;
/