Error when adding an item to HU using FM ADD_TO_EXISTING_HU

Question

Please I need help, could some tell me what I am doing wrong I keep an error when I try to add an item to a HU, here is the code:
FUNCTION zrf_hu_add_pack.
""Local Interface:
*"  IMPORTING
*"     VALUE(HU_HEADER) LIKE  BAPIHUHDRPROPOSAL STRUCTURE
*"        BAPIHUHDRPROPOSAL
*"     VALUE(USERNAME) LIKE  SY-UNAME OPTIONAL
*"     VALUE(PRINTER) TYPE  RSPOLNAME OPTIONAL
*"  EXPORTING
*"     VALUE(RETURN) LIKE  BAPIRET2 STRUCTURE  BAPIRET2
*"  TABLES
*"      HU_ITEMS STRUCTURE  BAPIHUITMPROPOSAL
  DATA: ret TYPE STANDARD TABLE OF bapiret2,
        header TYPE bapihuhdrproposal,
        hukey TYPE bapihukey-hu_exid,
        huheader TYPE bapihuheader,
        itemsproposal TYPE STANDARD TABLE OF bapihuitmproposal,
        itm TYPE STANDARD TABLE OF bapihuitem,
        item_prop TYPE  huitm_prop,
        obj TYPE hum_object,
        del_type TYPE vbtyp,
        hu_ext TYPE  hum_exidv_t,
        hu_int TYPE hum_venum_t,
        out_msg TYPE huitem_messages_t,
        tmp_vbeln TYPE vbeln,
        hu_hdr TYPE huhdr_proposal,
        hu_mm TYPE hum_plant_stloc,
        itm_wa LIKE LINE OF itm,
        item_prop_wa LIKE LINE OF item_prop,
        hui_wa LIKE LINE OF hu_items.
  DATA: ls_handling_units TYPE hum_rehang_hu,
        ls_high TYPE vekp-venum.
  DATA: it_items LIKE huitm_proposal,
         hm_obj TYPE hum_object,
         hu_prop TYPE huhdr_proposal,
         hu_add TYPE huhdr_add,
         hu_add_wa LIKE LINE OF hu_add,
         hu_items1 TYPE huitm_prop,
         hu_items_wa LIKE LINE OF hu_items1.
  DATA: hu_msg TYPE huitem_messages_t,
          hu_msg_wa LIKE LINE OF hu_msg,
          hu_hdr1 TYPE     hum_hu_header_t,
          hu_hdr1_wa LIKE LINE OF hu_hdr1.
  DATA: i_mm_data  LIKE  hum_plant_stloc,
        hu_internal TYPE  hum_venum_t,
        hu_external TYPE  hum_exidv_t,
        handling_unit TYPE  huitem_from,
        h_unit_l TYPE huitem_from,
        is_item_proposal TYPE  huitm_proposal,
        i_delivery_type  TYPE  likp-vbtyp ,
        et_messages      TYPE  huitem_messages_t.
  DATA: venum TYPE venum.
  DATA: lt_hu LIKE bapihunumber OCCURS 0 WITH HEADER LINE,
        lt_huhdr LIKE bapihuheader OCCURS 0 WITH HEADER LINE.
  DATA: t_return TYPE TABLE OF bapiret2.
get header
  lt_hu-hu_exid = hu_header-hu_exid.
  APPEND lt_hu.
get HU header
  CALL FUNCTION 'BAPI_HU_GETLIST'
    EXPORTING
      onlykeys      = ' '
    TABLES
      hunumbers     = lt_hu
      huheader      = lt_huhdr
      return        = t_return
    EXCEPTIONS
      error_message = 2
      OTHERS        = 4.
  IF sy-subrc NE 0.
    CALL FUNCTION 'BALW_BAPIRETURN_GET2'
      EXPORTING
        type   = sy-msgty
        cl     = sy-msgid
        number = sy-msgno
        par1   = sy-msgv1
        par2   = sy-msgv2
        par3   = sy-msgv3
        par4   = sy-msgv4
      IMPORTING
        return = return.
    ROLLBACK WORK.
  ELSE.
get defaults
    PERFORM get_user USING username.
handling unit
    READ TABLE lt_huhdr INDEX 1.
    APPEND hu_header-hu_exid TO hu_external.
    APPEND lt_huhdr-hu_id TO hu_internal.
    h_unit_l-exidv = lt_huhdr-hu_exid.
    h_unit_l-venum = lt_huhdr-hu_id.
APPEND h_unit_l to handling_unit.
hold delivery number
    tmp_vbeln = hu_header-pack_mat_customer.
convert or pad
    CALL FUNCTION 'CONVERSION_EXIT_ALPHA_INPUT'
      EXPORTING
        input  = tmp_vbeln
      IMPORTING
        output = tmp_vbeln.
set up object key
    obj-object = '01'.
    obj-objkey = tmp_vbeln.
    CONDENSE obj-objkey.
get delivery type from delivery header
    SELECT SINGLE vbtyp INTO del_type
    FROM likp
    WHERE vbeln = obj-objkey.
set status
    hu_header-hu_status_init = '1'.
    MOVE-CORRESPONDING hu_header TO hu_mm.
    hu_mm-plant = hu_header-plant.
    hu_mm-stge_loc = hu_header-stge_loc.
    hu_mm-whse_no = '101'.
    hu_mm-stge_type = '916'.
    hu_mm-stge_bin = tmp_vbeln.
set up items
    LOOP AT hu_items INTO hui_wa.
      hu_items_wa-velin = '1'.
      hu_items_wa-belnr = tmp_vbeln.
      hu_items_wa-posnr = hui_wa-lower_level_exid.
      MOVE hui_wa-pack_qty      TO hu_items_wa-quantity.
      MOVE hui_wa-base_unit_qty TO hu_items_wa-meins.
      CALL FUNCTION 'CONVERSION_EXIT_MATN1_INPUT'
        EXPORTING
          input        = hui_wa-material
        IMPORTING
          output       = hu_items_wa-matnr
        EXCEPTIONS
          length_error = 1
          OTHERS       = 2.
      hu_items_wa-charg = hui_wa-batch.
      hu_items_wa-werks = hui_wa-plant.
      hu_items_wa-lgort = hui_wa-stge_loc.
     APPEND hu_items_wa TO hu_items1.
sort items
   SORT hu_items1 BY belnr posnr.
copy and change function ADD_TO_EXISTING_HU if neccessary
      CALL FUNCTION 'ADD_TO_EXISTING_HU'
        EXPORTING
          i_hu_id               = ''
          i_hu_ref_doc          = obj
          i_mm_data             = hu_mm
          i_hu_internal         = hu_internal
          i_hu_external         = hu_external
          is_handling_unit      = h_unit_l
          is_item_proposal      = hu_items_wa
          i_delivery_type       = del_type
        IMPORTING
          et_messages           = hu_msg
        EXCEPTIONS
          not_possible          = 1
          delivery_update_error = 2
          OTHERS                = 3.
      IF sy-subrc <> 0.
        CALL FUNCTION 'BALW_BAPIRETURN_GET2'
          EXPORTING
            type   = sy-msgty
            cl     = sy-msgid
            number = sy-msgno
            par1   = sy-msgv1
            par2   = sy-msgv2
            par3   = sy-msgv3
            par4   = sy-msgv4
          IMPORTING
            return = return.
        ROLLBACK WORK.
      ELSE.
        COMMIT WORK AND WAIT.
      ENDIF.
    ENDLOOP.
  ENDIF.
ENDFUNCTION.

Answer 1

It probably has something to do wit the kind of HU that you are using.
If you call a " where used " of the error message HUFUNCTIONS 261 you will see that there are several checks concerning the kind of HU.
The types of HU's can be found in domain VPOBJ.
Hope this helps,
Regards,
Dirk.