As part of L3, we're not told what to name our modules. In chapter 5, you should have built a module that contains the actual sales, I just happened to name mine TAR10. Perhaps you named it differently than I did. Take another look at your modules, it should be there. Look for a module that has country and product family as lists.
That will work but it's not the most efficient way. The best way is to follow the DISCO methodology and use the system module, in this case SYS03. By putting the line items in TAR01 you are essentially calculating the same value over and over, placing a burden on the calculation engine, not to mention you'll take up more memory than necessary. By using the SYS03 module you calculate once, and the reference it from TAR01.
Hey, you're on the right track, first step is to get it to work. In L3 this is great time to explore how to optimize the model. Try using SYS03, you'll see it works great. You can use the formula I gave you above!