Hi Patrick,
You have inserted the roof on the Ground floor not the Second floor so it is below your walls.
Delete the roof, insert the walls on the second floor so they follow the walls of the ground floor and insert the roof again on the second floor. Make sure the roof "Solution Type" is set to "Ignore Next Storey" (or it may have a hole in roof).
Now you will be able to insert windows in the gable ends as in the image below.
Hope that solves it for you.
Allan