You need something like this:
=Sum({< Year, OrderDate = {">=$(=date(yearstart(addyears(max(OrderDate),-1)), 'DD/MM/YYYY'))<=$(=date(addyears(max(OrderDate),-1), 'DD/MM/YYYY'))"}>} Sales)
whereby your trouble is caused from different formatting. Your OrderDate is 'DD/MM/YYYY' but your default format for dates within the application defined by the interpreting-variables at the script-start in tab Main is 'MM/DD/YYYY'.
IMO all dates and their interpretation within an application should have the same format und further I suggest to avoid all such trouble by using pure numeric values for each kind of matching and calculations. This means in your case just to double the OrderDate with an additionally floor(OrderDate) as OrderDateNum and then you could mostly use num() or floor() to call the comparing/calculation wherever you need it.
- Marcus
