check this one.

you cannot use a column from a table which you are using in inner query. That's not gonna work

I tested my query one more time with some test data and it works.

declare @Cust_Inv as
table
(
	cust_acc_No varchar(100),
	Inv_No varchar(30)
	);
declare @cust_det as
table
(
	order_id int,
	fname varchar(100),
	lname varchar(100),
	order_date date,
	ord_paid date,
	cust_acc_No varchar(100)
);
insert into @Cust_Inv values(1,1);
insert into @Cust_Inv values(2,2);
insert into @Cust_Inv values(3,3);
insert into @Cust_Inv values(4,4);
insert into @Cust_Inv values(5,5);
insert into @Cust_Inv values(6,6);
insert into @cust_det values(1,'first name 1','last name 1',GETDATE()-1,GETDATE()-2,1);
insert into @cust_det values(2,'first name 2','last name 2',GETDATE()-3,GETDATE()-4,2);
insert into @cust_det values(3,'first name 3','last name 3',GETDATE()-5,GETDATE()-6,3);
insert into @cust_det values(4,'first name 4','last name 4',GETDATE()-7,GETDATE()-8,4);
insert into @cust_det values(5,'first name 5','last name 5',GETDATE()-9,GETDATE()-10,5);
insert into @cust_det values(6,'first name 6','last name 6',GETDATE()-11,GETDATE()-12,6);
select distinct
       top 6
       order_id as order_id,
       invoices.Inv_No,
       fname as fname ,
       lname as lname,
       order_date as order_date,
       ord_paid as ord_paid
  from @Cust_Inv invoices,
       @cust_det details
where invoices.Cust_Acc_No=details.Cust_Acc_No
order by invoices.INV_No

and output is this

[URL=http://imagefra.me/][IMG]http://img37.imagefra.me/img/img37/8/8/14/dsanthoshece/f_pfhnfps54gjm_4c8a64c.png[/IMG][/URL]

I don't know why are getting errors if you are using the similar query