Oracle GROUP BY issue I have this query:

Select To_Char(Order.Date, 'IW') As "Week No",
       Order.User As "User",
       Count(Order.OrderNo) As "Total Orders",
       Sum(Order_Totals.Qty) As "Total Items",
From Order Order,
     Order_Totals Order_Totals
Where Order.OrderNo = Order_Totals.OrderNo
And (Order.Date Between '01-JAN-15' And '31-DEC-15')
Group By Order.User


But it throws the error message:

Not a valid GROUP BY expression


I want to show how many orders and items each user has created each week this year, looking at http://www.techonthenet.com/oracle/group_by.php I thought that I am using GROUP BY correctly, but clearly not and I am not sure how I can sort it.
You have to add To_Char(Order.Date, 'IW') in the GROUP BY too.