It might not work within your actual data because of many possible reasons. You might have multiple years data. If so, you have to consider the year field as well or even only the date field. For multiple years data try like:
Sum({<Year={$(=Max(Year))}, [DataWeek] = {$(=Max({<[Transaction Date]={"$(=Max([Transaction Date]))"}>} DataWeek))}, Month = {$(=text(Month(Max([Transaction Date]))))}>} InvQty)
Otherwise, try to grab the logic why we used such expression and try to resolve yourself or prepare a sample with sample data with all possible scenario you might have in actual data - to let us work on that.
