ERROR : You cannot use an internal table as a work area

Hi, I am new to ABAP .

My Use case is,

I want to get all "not processed " ,"Partially processed" and "Completed" orders for a given Customer Number.

For that,  I am selecting some fields of "VBAK" table on basis of "KUNNR" and JOINING "VBUK" table on "VBLEN".

In "VBUK" i am using "GBSTK" to check the order status.

I want to return this list from this Function Module.

Im using  "into  it_vbak" clause, the field order in select statement and the order in structure is same.

I have written folloing code.

FUNCTION ZCUSTOMER_ORDER.
*"----------------------------------------------------------------------
*"*"Local Interface:
*"  EXPORTING
*"     REFERENCE(ORDERS) TYPE  ZTABLECUSTALLORDER
*"----------------------------------------------------------------------
TABLES:VBAK,VBUK.


DATA: it_vbak TYPE TABLE OF  ZALLORDERS INITIAL SIZE 100,
        wa_vbak TYPE ZALLORDERS.


*DATA: it_vbak like standard table of ZALLORDERS,
*      wa_vbak TYPE it_vbak.

*DATA: ty_VBAK1 like standard table of ty_VBAK with header line with key VBLEN.
*DATA: wa_VBAK like Line of ty_VBAK1.

select  VBAK~VBELN
         VBAK~ERNAM
         VBAK~ANGDT
         VBAK~BNDDT
   VBAK~BSTNK
    VBAK~BSTDK
    VBAK~TELF1
   VBAK~NETWR
   VBAK~WAERK
   VBUK~GBSTK

  into  it_vbak
   from VBAK
join VBUK on ( VBAK~VBELN = VBUK~VBELN ) where
VBUK~GBSTK in ('A','B','C') and  VBAK~KUNNR = '0000000001'.

loop at it_vbak into  wa_vbak.
    WRITE:/ wa_vbak-vblen,
            wa_vbak-angdt,
             wa_vbak-bnddt.


endloop.

*READ TABLE it_vbak INTO ORDERS.

ENDFUNCTION.

The  "ZTABLECUSTALLORDER" is my export type from Function module, This is a table of structure (Line Type) .

The structure is ZALLORDERS, The components are displayed in following Image.

When i am trying to compile this code, it is giving "

You cannot use an internal table as a work area

" error.

Could some one please help me, what i am doing which is incorrect here.

Thanks in advance