Hi Gopi ,
Change the order of appending wa_fldcat to lt_fldcat .Also pass the values to COL_POS in single quotes .
append in this order :
WA_FLDCAT-FIELDNAME = 'TICK_NO'.
WA_FLDCAT-TABNAME = 'ZWEIGHT_HEADER'.
WA_FLDCAT-SELTEXT_L = 'TICKET NO'.
WA_FLDCAT-SELTEXT_M = 'TICKET NO'.
WA_FLDCAT-SELTEXT_S = 'TICKET NO'.
WA_FLDCAT-OUTPUTLEN = 15.
WA_FLDCAT-COL_POS = '1'.
APPEND WA_FLDCAT TO LT_FLDCAT.
CLEAR WA_FLDCAT.
WA_FLDCAT-FIELDNAME = 'SNO'.
WA_FLDCAT-TABNAME = 'ZWEIGHT_ITEM'.
WA_FLDCAT-SELTEXT_L = 'SERIAL NUMBER'.
WA_FLDCAT-SELTEXT_M = 'SNO'.
WA_FLDCAT-SELTEXT_S = 'SNO'.
WA_FLDCAT-OUTPUTLEN = 5.
WA_FLDCAT-COL_POS = '2' .
APPEND WA_FLDCAT TO LT_FLDCAT.
CLEAR WA_FLDCAT.
WA_FLDCAT-FIELDNAME = 'FISRT_WT'.
WA_FLDCAT-TABNAME = 'ZWEIGHT_HEADER'.
WA_FLDCAT-SELTEXT_L = 'FIRST WEIGHT'.
WA_FLDCAT-SELTEXT_M = 'FIRST WEIGHT'.
WA_FLDCAT-SELTEXT_S = 'FIRST WEIGHT'.
WA_FLDCAT-OUTPUTLEN = 15.
WA_FLDCAT-COL_POS = '3'.
APPEND WA_FLDCAT TO LT_FLDCAT.
CLEAR WA_FLDCAT.
WA_FLDCAT-FIELDNAME = 'SECOND_WT'.
WA_FLDCAT-TABNAME = 'ZWEIGHT_HEADER'.
WA_FLDCAT-SELTEXT_L = 'SECOND WEIGHT'.
WA_FLDCAT-SELTEXT_M = 'SECOND WEIGHT'.
WA_FLDCAT-SELTEXT_S = 'SECOND WEIGHT'.
WA_FLDCAT-OUTPUTLEN = 15.
WA_FLDCAT-COL_POS = '4'.
APPEND WA_FLDCAT TO LT_FLDCAT.
CLEAR WA_FLDCAT.
WA_FLDCAT-FIELDNAME = 'NET_WEIGHT'.
WA_FLDCAT-TABNAME = 'ZWEIGHT_HEADER'.
WA_FLDCAT-SELTEXT_L = 'NET_WEIGHT'.
WA_FLDCAT-SELTEXT_M = 'NET_WEIGHT'.
WA_FLDCAT-SELTEXT_S = 'NET_WEIGHT'.
WA_FLDCAT-OUTPUTLEN = 15.
WA_FLDCAT-COL_POS = '5'.
APPEND WA_FLDCAT TO LT_FLDCAT.
CLEAR WA_FLDCAT.
WA_FLDCAT-FIELDNAME = 'F_DATE'.
WA_FLDCAT-TABNAME = 'ZWEIGHT_HEADER'.
WA_FLDCAT-SELTEXT_L = 'FIRST DADE'.
WA_FLDCAT-SELTEXT_M = 'FIRST DADE'.
WA_FLDCAT-SELTEXT_S = 'FIRST DADE'.
WA_FLDCAT-OUTPUTLEN = 15.
WA_FLDCAT-COL_POS = '6'.
APPEND WA_FLDCAT TO LT_FLDCAT.
CLEAR WA_FLDCAT.
WA_FLDCAT-FIELDNAME = 'SEC_DATE'.
WA_FLDCAT-TABNAME = 'ZWEIGHT_HEADER'.
WA_FLDCAT-SELTEXT_L = 'SECOND DATE'.
WA_FLDCAT-SELTEXT_M = 'SECOND DATE'.
WA_FLDCAT-SELTEXT_S = 'SECOND DATE'.
WA_FLDCAT-OUTPUTLEN = 15.
WA_FLDCAT-COL_POS = 7.
APPEND WA_FLDCAT TO LT_FLDCAT.
CLEAR WA_FLDCAT.