Push-/pull doesn't work to open windows in walls

I’ve been trying to build a house based on a floor plan. I followed a tutorial online (SketchUp Interior Design for Layout Part 2 - Doors and Windows - YouTube) but I encounter a problem when adding windows to the walls. I am able to add the window frames but then i cant push/pull the wall to remove it so i can see through the windows. I tried a lot but it just does not seems to work… I hope somebody can help me because the house is amost finished!
Greetings, Riemke

Raadwijk 3bis.skp (8.7 MB)

You need to open the group for editing before you can cut the hole.
Note the solid blue bounding box changes to a dotted line when in edit mode.

Thank you for your help! That was exactly what I needed to know, now I can finish my house.