find this error when running - Only one expression can be specified in the select list when the subquery is not introduced with EXISTS.

I think this is what you are after:

SELECT * FROM
(
	select 
	pro.[Finished Date] finished_date,
	min(h.[Spcl Order]) Spcl_Order,
	case when sum(pro.[Quantity]) is null then 0 else sum(pro.[Quantity]) END pro_qty
	from [tablename1] pro 
	inner join [sample].[dbo].[Spcl Order_PO] as h on  Pro.[Spcl Order]=h.[Spcl Order]
	where ((pro.[Status]=4) and (pro.[Finished Date] ='2013-10-10' ))  
	group by pro.[Finished Date]
) as pro1
INNER JOIN
(
	select 
	 A.[Posting Date] posting_date
	 case when sum(A.[Quantity]) is null then 0 else sum(A.[Quantity]) END other_qty,
	 min(b.[Posting Date]) min_posting_date,
	 min(c.[Order No]) min_order,
	 min(d.[Prod_Order No]) min_prod_order,
	 min(e.[No]) min_no,
	 min(f.[Item Category Code]) min_cat_code,
	 min(g.[Location Code]) min_loc_code,
	 min(i.[Spcl Order_Purch]) min_order_purch
	from [tablename2] A 
	inner join [sample].[dbo].[Posting Date_PRL] as b on a.[Posting Date]=b.[Posting Date]
	inner join [sample].[dbo].[Order No_PRL] as c on a.[Order No_]=c.[Order No]
	inner join [sample].[dbo].[Prod_ OrderNo_PRL] as  d on a.[Prod_ Order No_]=d.[Prod_OrderNo] 
	inner join [sample].[dbo].[No_PRL] as e on a.[No_]=e.[No]
	inner join [sample].[dbo].[Item Category Code_PRL] as f on a.[Item Category Code]=f.[Item Category Code]
	inner join [sample].[dbo].[Location Code_PRL] as g  on a.[Location Code]=g.[Location Code]
	inner join [sample].[dbo].[Spcl Order_PRL] as i on a.[Spcl Order]=i.[Spcl Order_Purch]
	where (A.[Item Category Code] = 'STYLES') and (A.[Prod_ Order No_]='') and ( A.[Buy-from Vendor No_] in ('S02052')) AND (A.[Quantity]>0) 
	GROUP BY A.[Posting Date])
)  as post
ON pro1.finished_date = post.posting_date

BTW, you still could use your query if you leave only select (case when sum(A.[Quantity]) is null then 0 else sum(A.[Quantity]) END) in your sub-query, and remove all other aggregates (min(b.[Posting Date]),min(c.[Order No]),min(d.[Prod_Order No]),min(e.[No]),
min(f.[Item Category Code]),min(g.[Location Code]),min(i.[Spcl Order_Purch])), i.e., this query will also work:

select 
min(h.[Spcl Order]),
case when sum(pro.[Quantity]) is null then 0 else sum(pro.[Quantity]) END
+
(select (case when sum(A.[Quantity]) is null then 0 else sum(A.[Quantity]) END)
 from [tablename2] A 
inner join [sample].[dbo].[Posting Date_PRL] as b on a.[Posting Date]=b.[Posting Date]
inner join [sample].[dbo].[Order No_PRL] as c on a.[Order No_]=c.[Order No]
inner join [sample].[dbo].[Prod_ OrderNo_PRL] as  d on a.[Prod_ Order No_]=d.[Prod_OrderNo] 
inner join [sample].[dbo].[No_PRL] as e on a.[No_]=e.[No]
inner join [sample].[dbo].[Item Category Code_PRL] as f on a.[Item Category Code]=f.[Item Category Code]
inner join [sample].[dbo].[Location Code_PRL] as g  on a.[Location Code]=g.[Location Code]
inner join [sample].[dbo].[Spcl Order_PRL] as i on a.[Spcl Order]=i.[Spcl Order_Purch]
where ((A.[Item Category Code] = 'STYLES') and (A.[Prod_ Order No_]='') and ( A.[Buy-from Vendor No_] in ('S02052')) AND (A.[Quantity]>0) 
AND ( A.[Posting Date] = pro.[Finished Date])))
 
from [tablename1] pro 
inner join [sample].[dbo].[Spcl Order_PO] as h on  Pro.[Spcl Order]=h.[Spcl Order]
where ((pro.[Status]=4) and (pro.[Finished Date] ='2013-10-10' ))  group by pro.[Finished Date]