Firstly to get these into a single result set you need both queries to return the same number of columns in the result.
CASE [
^]is the construct you need, not IF
Try this (untested)
SELECT a.PRODUCT_NO,b.PRODUCT_NAME,a.QTY_IN,
QTY_OUT = CASE WHEN a.WH_NO=b.WH_NO THEN b.QTY_OUT ELSE 0 END
,a.UNIT_COST
FROM STOCKDETAILS a,
INNER JOIN STOCKDETAILS b ON a.PRODUCT_NO=b.PRODUCT_NO AND b.STOCK_JR='OU'
WHERE a.WH_NO='WH1' AND a.STOCK_JR='IN'