字段符号未分配错误。无法理解我在Hierarchical报告程序中犯的错误

Question

这是一个层级报告，我正面临挑战。我无法弄清楚出了什么问题，我已经阅读了所有相关的博客，但是徒劳无功。请帮助我。

以下是代码：

REPORT  ZTEST28.

TYPE-POOLS: SLIS.

TYPES: BEGIN OF TY_EKKO,
        EBELN TYPE EBELN,
        STATU TYPE STATU,
        BEDAT TYPE BEDAT,
       END OF TY_EKKO,

       BEGIN OF TY_EKPO,
        EBELN TYPE EBELN,
        EBELP TYPE EBELP,
        TXZ01 TYPE TXZ01,
        MATNR TYPE MATNR,
       END OF TY_EKPO,

       TY_LAYOUT TYPE SLIS_LAYOUT_ALV.

DATA: IT_EKKO TYPE TABLE OF TY_EKKO,
      IT_EKPO TYPE TABLE OF TY_EKPO.


DATA: WA_EKKO TYPE TY_EKKO,
      WA_EKPO TYPE TY_EKPO,
      WA_LAYOUT TYPE ty_layout.

DATA: IT_FCAT TYPE SLIS_T_FIELDCAT_ALV,
      WA_FCAT TYPE SLIS_FIELDCAT_ALV,
      GS_KEYINFO TYPE SLIS_KEYINFO_ALV.

DATA: G_HEADER TYPE SLIS_TABNAME,
      G_ITEM TYPE SLIS_TABNAME.

SELECT-OPTIONS: S_EBELN FOR WA_EKKO-EBELN.

INITIALIZATION.
S_EBELN-LOW = '4500000000'.
S_EBELN-HIGH = '4500000010'.
S_EBELN-SIGN = 'I'.
S_EBELN-OPTION = 'BT'.
APPEND S_EBELN.

START-OF-SELECTION.

  SELECT EBELN STATU BEDAT FROM EKKO
    INTO TABLE IT_EKKO
    WHERE EBELN IN S_EBELN.

  IF SY-SUBRC = 0.
    SELECT EBELN EBELP TXZ01 MATNR FROM EKPO
      INTO TABLE IT_EKPO
      FOR ALL ENTRIES IN IT_EKKO
      WHERE EBELN EQ IT_EKKO-EBELN.
  ENDIF.

Data: LV_REPID LIKE SY-REPID.

WA_LAYOUT-COLWIDTH_OPTIMIZE = 'X'.
WA_LAYOUT-MIN_LINESIZE = 1.
  wa_layout-zebra = 'X'.

WA_FCAT-FIELDNAME = 'EBELN'.
WA_FCAT-SELTEXT_L = 'Pur Doc No'.
APPEND WA_FCAT TO IT_FCAT.
CLEAR WA_FCAT.

WA_FCAT-FIELDNAME = 'STATU'.
WA_FCAT-SELTEXT_L = 'Status'.
APPEND WA_FCAT TO IT_FCAT.
CLEAR WA_FCAT.

WA_FCAT-FIELDNAME = 'BEDAT'.
WA_FCAT-SELTEXT_L = 'Date'.
APPEND WA_FCAT TO IT_FCAT.
CLEAR WA_FCAT.

WA_FCAT-FIELDNAME = 'EBELP'.
WA_FCAT-SELTEXT_L = 'Pur Item No'.
APPEND WA_FCAT TO IT_FCAT.
CLEAR WA_FCAT.

WA_FCAT-FIELDNAME = 'TXZ01'.
WA_FCAT-SELTEXT_L = 'Tax Code'.
APPEND WA_FCAT TO IT_FCAT.
CLEAR WA_FCAT.

WA_FCAT-FIELDNAME = 'MATNR'.
WA_FCAT-SELTEXT_L = 'Material'.
APPEND WA_FCAT TO IT_FCAT.
CLEAR WA_FCAT.

GS_KEYINFO-HEADER01 = 'EBELN'.
GS_KEYINFO-ITEM01 = 'EBELN'.

G_HEADER = 'IT_EKKO'.
G_ITEM = 'IT_EKPO'.

CALL FUNCTION 'REUSE_ALV_HIERSEQ_LIST_DISPLAY'
  EXPORTING
   I_CALLBACK_PROGRAM             = SY-CPROG
   IS_LAYOUT                      = WA_LAYOUT
   IT_FIELDCAT                    = IT_FCAT
    i_tabname_header               = G_HEADER
    i_tabname_item                 = G_ITEM
    is_keyinfo                     = GS_KEYINFO

  tables
    t_outtab_header                = IT_EKKO
    t_outtab_item                  = IT_EKPO
 EXCEPTIONS
   PROGRAM_ERROR                  = 1
   OTHERS                         = 2
          .
IF sy-subrc <> 0.
* MESSAGE ID SY-MSGID TYPE SY-MSGTY NUMBER SY-MSGNO
*         WITH SY-MSGV1 SY-MSGV2 SY-MSGV3 SY-MSGV4.
ENDIF.

------＆GT;在调用功能模块的最后，我面临这个错误。