Official comment
Thank you for contacting us. I suspect the issue is because you set your below grade room height to 9ft, but also specified the entire 9ft is below ground. I believe the system will not allow a window to be 100% underground (which makes sense). If you change the below grade height to be less than the room height it should allow you to add windows. You can also go into the wall you added and hit the advanced edit icon which will allow you to edit the below grade height of that specific wall.
Best Regards
Bill