Have you watched the tutorials? I would 100% watching our tutorials first. I would start off by watching all the beginner tutorials.
Check them out here - https://plusspec.com/tutorials-splash.html
The reason you got a "red error" in your dialogue box is because you either have your header height larger than the wall height or your window height is larger than your header height.
The magenta line is meant to be there and if you click on the wall with the magenta line you will see that the magenta line appears on the wall and another appears, select another point and you will see the window appear.
I would advise to not touch layers and scenes until you completely understand them. They are set up the way they are for a reason, if you have better ideas you can experiment but I wouldn't experiment with a project, I would experiment in a new test file until I find exactly what I want and then apply to my project model. Please also email us your layers and scene set up and we could even implement it as normal in our software potentially.