Try following code, It will work

FUNCTION zewm_f_cancel_pick.
*”———————————————————————-
*”*”Local Interface:
*” IMPORTING
*” VALUE(IV_LGNUM) TYPE /SCWM/LGNUM
*” VALUE(IT_DOCID) TYPE /SCWM/DLV_DOCID_ITEM_TAB
*” VALUE(IV_QNAME) TYPE TRFCQNAM
*”———————————————————————-
WAIT UP TO 2 SECONDS.
DATA: lo_hu_mon TYPE REF TO zcl_ewm_cancpick_mon.
DATA: lv_noexecute TYPE sy-input.
DATA: lt_ret TYPE bapirettab.
DATA: lv_msg TYPE msgtx.

*–Append message for queue start
MESSAGE s171 INTO lv_msg.
PERFORM add_message TABLES lt_ret
USING lv_msg.

*–Get HU List
TRY.
DATA(lo_dlv) = /scwm/cl_dlv_management_prd=>get_instance( ).
DATA(ls_read_options) = VALUE /scwm/dlv_query_contr_str( data_retrival_only = abap_true ). “lock_result = abap_true
DATA(ls_include) = VALUE /scwm/dlv_query_incl_str_prd( head_status = abap_true ).
lo_dlv->query(
EXPORTING
iv_doccat = /scdl/if_dl_c=>sc_doccat_out_prd
it_docid = it_docid
is_read_options = ls_read_options
is_include_data = ls_include
IMPORTING
et_headers = DATA(lt_headers)
et_hu_headers = DATA(lt_hu_headers)
et_hu_items = DATA(lt_hu_items)
eo_message = DATA(lo_message) ).

CATCH /scdl/cx_delivery INTO DATA(lx_dlv).

ENDTRY.

*–Cancelpick Delivery
lo_hu_mon = zcl_ewm_cancpick_mon=>get_instance( ).

lo_hu_mon->/scwm/if_mon_hu~cancel_hu_pick(
EXPORTING
iv_lgnum = iv_lgnum
it_data = lt_hu_headers
).

DATA(lt_return) = lo_hu_mon->get_message( ).

*–Append message for queue ending
MESSAGE s172 INTO lv_msg.
PERFORM add_message TABLES lt_ret
USING lv_msg.

*–form return table for the queue
IF lt_ret[] IS NOT INITIAL.
APPEND LINES OF lt_ret TO lt_return.
ENDIF.

IF VALUE #( lt_return[ type = ‘E’ ]-type OPTIONAL ) = ‘E’.
/spe/cl_qrfc_services=>end_queue_processing(
EXPORTING
it_message = lt_return ” Table with BAPI Return Information
).
ENDIF.
/spe/cl_qrfc_services=>end_queue_processing(
EXPORTING
it_message = lt_return ” Table with BAPI Return Information
).

*create Queue FM for update
CALL FUNCTION ‘TRFC_SET_QIN_PROPERTIES’
EXPORTING
qin_name = iv_qname
no_execute = lv_noexecute
EXCEPTIONS
invalid_queue_name = 1
OTHERS = 2.

CALL FUNCTION ‘ZEWM_F_CANCEL_PICK_MOVE’
IN BACKGROUND TASK AS SEPARATE UNIT
DESTINATION ‘NONE’
EXPORTING
iv_lgnum = iv_lgnum
it_hu_hdr = lt_hu_headers
it_hu_itm = lt_hu_items.

COMMIT WORK AND WAIT.

/spe/cl_qrfc_services=>end_queue_processing(
EXPORTING
it_message = lt_return ” Table with BAPI Return Information
).
ENDFUNCTION.

Here is the sample code to read HU

Create Instance for /scwm/cl_wm_packing class

*--Get docid from the source HU
      CALL METHOD /scwm/cl_wm_packing=>get_instance
        IMPORTING
          eo_instance = DATA(lo_wm_pack).



* --init warehouse,application
      CALL METHOD lo_wm_pack->init_pack
        EXPORTING
          iv_badi_appl = 'WME'
          iv_lgnum     = is_ltap-lgnum
        EXCEPTIONS
          error        = 1
          OTHERS       = 2.



* --get hu data, in this case only item data
      CALL METHOD lo_wm_pack->get_hu
        EXPORTING
          iv_guid_hu = is_ltap-sguid_hu                 " Unique Internal Identification of a Handling Unit
        IMPORTING
          et_huitm   = DATA(lt_huitm)                   " Material Items in the HU
        EXCEPTIONS
          not_found  = 1
          OTHERS     = 2.