Looking for a bit of help.

I am receiving the below error when trying to count the results of a case statement involving a sum.

I am trying to find some workaround as I know this is not the correct way to do it but I am totally stumped.

In English I want to:

Count the amount of orders that has a total item quantity of 1.

The SQL I am using is:

,COUNT(CASE
WHEN SUM(delivery_line_item.dli_qty) = 1 THEN
1
ELSE
0
END)
AS SingleLineOrders

Without the Count statement, the query returns the result of 1, where I need to count all instances where the sum of dli_qty = 1.

Hopefully someone can help 🙂

Thanks