Problems modelling Excelfile for use in PBI

Jackofall 

OK, here is how I would go about it. It is a different method to yours though...

1) Unpivot the data on import: select the period and date columns and under unpivot choose "unpivot other columns"

2) Add a conditional column to create a new column classifying categories as right or wrong:

and 

Your Fact table should now look like this (I´ve actually deleted the period column you had, since I always include it in my calendar table):

3) Create a Calendar Table

4) I've also created a Category Table by referencing from the fact table, deleting all columns except the Category and Right or Wrong Columns, and deleted duplicate values. The table looks like this:

5) Load all into the model and create the relevant relationships between tables. The model should resemble  this:

6) Create any measures you need for the pie chart; I've just made a simple sum of values.

7) Create your pie chart using the "Right or Wrong" and "Category" fields from your Category Table as your legend and your measure as values:

You can now drill down/up etc on your pie chart (by right-clicking on a segment or using the drill-triggers on the chart):

and once you have "drilled-down":

And here is the PBIX file if you're interested:
Pie Chart Example 

Hope it helps, and let us know if something isn't working for you!