sql query giving me an error

Hi salinis Giving an alias to the table which is returned by subquery should resolve this error. Try the below: 

SELECT DISTINCT
A.ITS_MATTER_NUMBER,
A.REQUEST_DATE,
A.REQUEST_TYPE,
A.REQUEST_STATUS,
A.CUSTODIAN_NAME,
A.CUSTODIAN_WWID,
A.JNJ_MATTER_NAME,
A.JNJ_MATTER_NUMBER,
A.COLLECTION_VENDOR,
A.RECEIVER
FROM
(
SELECT DISTINCT
ITS_MATTER_NO AS ITS_MATTER_NUMBER,
TO_CHAR(CREATE_DATE,'MM/DD/YYYY') AS REQUEST_DATE,
REQUEST_TYPE,
CFA_DM_ED_REQUEST_HDR.STATUS AS REQUEST_STATUS,
CUSTODIAN_NAME,
CUSTODIAN_WWID,
JNJ_MATTER_NAME,
JNJ_MATTER_NO AS JNJ_MATTER_NUMBER,
CFA_DM_ED_COLL_VENDOR.COMPANY AS COLLECTION_VENDOR,
CFA_DM_ED_PROD_RECEIVER.COMPANY AS RECEIVER
FROM CFA_DM_ED_REQUEST_HDR
LEFT OUTER JOIN CFA_DM_ED_PROD_RECEIVER ON CFA_DM_ED_REQUEST_HDR.REQUEST_ID = CFA_DM_ED_PROD_RECEIVER.REQUEST_ID_FK
LEFT OUTER JOIN CFA_DM_ED_COLL_VENDOR ON CFA_DM_ED_REQUEST_HDR.REQUEST_ID = CFA_DM_ED_COLL_VENDOR.REQUEST_ID_FK
JOIN
(
SELECT
CFA_DM_ED_LIVE_CUSTODIAN.FIRST_NAME || ' ' || CFA_DM_ED_LIVE_CUSTODIAN.LAST_NAME AS CUSTODIAN_NAME,
CFA_DM_ED_LIVE_CUSTODIAN.WWID AS CUSTODIAN_WWID,
REQUEST_ID_FK
FROM CFA_DM_ED_LIVE_CUSTODIAN
UNION ALL
SELECT
CFA_DM_ED_HISTORIC_CUSTODIAN.FIRST_NAME || ' ' || CFA_DM_ED_HISTORIC_CUSTODIAN.LAST_NAME AS CUSTODIAN_NAME,
CFA_DM_ED_HISTORIC_CUSTODIAN.WWID AS CUSTODIAN_WWID,
REQUEST_ID_FK
FROM CFA_DM_ED_HISTORIC_CUSTODIAN
ORDER BY REQUEST_ID_FK
) T1 ON CFA_DM_ED_REQUEST_HDR.REQUEST_ID = T1.REQUEST_ID_FK
ORDER BY REQUEST_ID,ITS_MATTER_NO) A

This is an optimized query where I have Removed DISTINCT from the inner query since union and proper joins ensure unique results.
SELECT DISTINCT
A.ITS_MATTER_NUMBER,
A.REQUEST_DATE,
A.REQUEST_TYPE,
A.REQUEST_STATUS,
A.CUSTODIAN_NAME,
A.CUSTODIAN_WWID,
A.JNJ_MATTER_NAME,
A.JNJ_MATTER_NUMBER,
A.COLLECTION_VENDOR,
A.RECEIVER
FROM (
SELECT
HDR.ITS_MATTER_NO AS ITS_MATTER_NUMBER,
TO_CHAR(HDR.CREATE_DATE, 'MM/DD/YYYY') AS REQUEST_DATE,
HDR.REQUEST_TYPE,
HDR.STATUS AS REQUEST_STATUS,
CUST.CUSTODIAN_NAME,
CUST.CUSTODIAN_WWID,
HDR.JNJ_MATTER_NAME,
HDR.JNJ_MATTER_NO AS JNJ_MATTER_NUMBER,
VENDOR.COMPANY AS COLLECTION_VENDOR,
RECEIVER.COMPANY AS RECEIVER
FROM CFA_DM_ED_REQUEST_HDR HDR
-- Join to Production Receiver table
LEFT JOIN CFA_DM_ED_PROD_RECEIVER RECEIVER
ON HDR.REQUEST_ID = RECEIVER.REQUEST_ID_FK
-- Join to Collection Vendor table
LEFT JOIN CFA_DM_ED_COLL_VENDOR VENDOR
ON HDR.REQUEST_ID = VENDOR.REQUEST_ID_FK
-- Subquery to fetch Custodian information
LEFT JOIN (
SELECT
FIRST_NAME || ' ' || LAST_NAME AS CUSTODIAN_NAME,
WWID AS CUSTODIAN_WWID,
REQUEST_ID_FK
FROM (
-- Union Live and Historic Custodian tables
SELECT
LIVE.FIRST_NAME,
LIVE.LAST_NAME,
LIVE.WWID,
LIVE.REQUEST_ID_FK
FROM CFA_DM_ED_LIVE_CUSTODIAN LIVE
UNION ALL
SELECT
HIST.FIRST_NAME,
HIST.LAST_NAME,
HIST.WWID,
HIST.REQUEST_ID_FK
FROM CFA_DM_ED_HISTORIC_CUSTODIAN HIST
) CUSTODIANS
) CUST
ON HDR.REQUEST_ID = CUST.REQUEST_ID_FK
) A;