So, you are not there, but we can get you there. When you are doing a sum, the line item has to be a number (like you have in Sales), but the what you are summing on has to be a list formatted line item (the Product or Consumer, in your picture). If these are Text formatted, then they will not work. So, there are two ways around this:
The ideal way is make the Product and Consumer list formatted to the correct lists (presumably Product Flat and Consumer Flat) or
keep them as text, and create two line items
Product List: Finditem(Product Flat, Product)
Consumer LIst: Finditem(Consumer Flat, Consumer)
Now your formula in the target module (if it is dimensionalized by Product Flat would be: Sum Source Module.Sales[sum: Product] or if you did it the 2nd way, Sum Source Module.Sales[sum: Product List]. If you have both Consumer Flat and Product Flat in your target module, the formula would be:
First way: Sum Source Module.Sales[sum: Product, sum Consumer]
Second way: Sum Source Module.Sales[sum: Product List, sum Consumer List]
Just remember, when you are doing a sum, the line items within the brackets have to be formatted as a List.
I am confused because your last picture has all of it done correctly...You are summing the Sales data from the Source Module by two different list formatted line items (Product and Consumer). If you wanted just the Sales data by Products, then your Result Module would dimensionalized by Product with your formula being Source Module.Sales[Sum: Product].
Sales is a line item as well as Product and Consumer on the the Transaction List, essentially metadata about that transactional line (think invoice ID). What the exercise is wanting you to do is to sum up the all sales by the product as well as the consumer, not the transactional ID. When using a sum, the items between the brackets ([ ]) have to be dimensions of the target module defined either on the rows, columns, or page axis (what your are wanting to sum by). Since your target module does not have the Transactional list on the"outside" of the house (rows, columns, or page axis), the sum will not work with the transactional list as part of the sum function.