Hi,
I'd say the subquery it doesn't like is the
AND Products.Cust_ID = (SELECT Customers.Cust_ID FROM Customers WHERE Customers.Bill_Cycle_ID = "1")
bit. It's likely that this select returns more than one value so what you are saying is something like -
AND Products.Cust_ID = (1,10,12,15,19,32,etc)
You instead want to achieve something like
AND Products.Cust_ID IN (1,10,12,etc)