You are missing INV_No column in SELECT. use the one below:

select * from
	(select top 6 * from
	(select top 220 order_id as order_id,fname as fname ,
		lname as lname, order_date as order_date,
		ord_paid as ord_paid from Cust_Inv ,cust_det, INV_No
		where cust_det.Cust_Acc_No=Cust_InV.Cust_Acc_No
		order by Cust_Inv.INV_No) as tbl1
		order by INV_No desc) as tbl2
		order by Cust_Inv.INV_No