HI I WANT TO BUILT A REPORT IN ORACLE DISCOVERER USING BELOW QUERY BUT I'M FACING ERROR PLEASE HELP

SQL QUERY:
SELECT OST_COMP_CODE,OST_MAIN_ACNT_CODE,OST_SUB_ACNT_CODE,ost_divn_code,ost_dept_code,OST_TRAN_CODE,OST_DOC_NO,OST_DOC_DT,
NVL(NVL(SUM(DECODE(OST_DRCR_FLAG,'D',NVL(OST_LC_ORG_AMT,0) - (F_GET_BASE_LC_AMT(OST_KEY_NO,'P',to_date**('&da','mm/dd/yyyy'))** +
F_GET_BASE_CROSS_LC_AMT(OST_KEY_NO,'C',to_date('&da','mm/dd/yyyy')) + F_GET_BASE_LC_AMT(OST_KEY_NO,'F',to_date**('&da','mm/dd/yyyy')**)))),0) -
NVL(SUM(DECODE(OST_DRCR_FLAG,'C',NVL(OST_LC_ORG_AMT,0) - (F_GET_BASE_LC_AMT(OST_KEY_NO,'P',to_date**('&da','mm/dd/yyyy')**) +
F_GET_BASE_CROSS_LC_AMT(OST_KEY_NO,'C',to_date('&da','mm/dd/yyyy')) + F_GET_BASE_LC_AMT(OST_KEY_NO,'F',to_date('&da','mm/dd/yyyy'))))),0),0) BALANCE,
to_date('&da','mm/dd/yyyy')-trunc(OST_DOC_DT) "DAYS"
FROM FT_OS
WHERE OST_MAIN_aCNT_CODE='A2501'
and trunc(ost_doc_dt)<= to_date('&da','mm/dd/yyyy')
AND NVL(OST_LC_ORG_AMT,0) - (F_GET_BASE_LC_AMT(OST_KEY_NO,'P',to_date('&da','mm/dd/yyyy')) +
F_GET_BASE_CROSS_LC_AMT(OST_KEY_NO,'C',to_date('&da','mm/dd/yyyy')) +F_GET_BASE_CROSS_LC_AMT(OST_KEY_NO,'D',to_date('&da','mm/dd/yyyy'))+ F_GET_BASE_LC_AMT(OST_KEY_NO,'F',to_date('&da','mm/dd/yyyy')))>0
AND EXISTS (SELECT 'X'
FROM FM_CUSTOMER X
WHERE X.CUST_CODE = OST_SUB_ACNT_CODE
UNION
SELECT 'X'
FROM FM_SUPPLIER Y
WHERE Y.SUPP_CODE = OST_SUB_ACNT_CODE)
GROUP BY ost_comp_code,OST_MAIN_aCNT_CODE,OST_SUB_aCNT_CODE,to_date('&da','mm/dd/yyyy')-trunc(OST_DOC_DT) ,ost_divn_code,ost_dept_code,OST_TRAN_CODE,OST_DOC_NO,OST_DOC_DT
REGARDS
VIGNESH