Put A Window Into A wall?

I have a basic wall and have noticed that the boolean tool doesn’t seem to work when I use window generator 2 for some reason. I was initially going to use the boolean tool to fit my wall in however I get a message saying the operation can’t be done or something along those lines. Any idea what I can do to do get it to fit? I have heard the boolean shouldn’t really be used anyway, unsure if that is true? The issue is that the window is curved at the top which makes it very difficult to fit, any tips would be great thanks:)

Tab into edit made and use Ctrl+R to add in an edge loop. (I often use a cube to set my scale and drag my edge loops to line up before removing the cube) After the edge loops are in place, enter face mode (ctrl+tab+2) and delete the face that is where you want the window. Now jump to edge mode (ctrl+tab+3) and Alt+click the edge of the opening and extrude the sills.

If you think about its not so difficult at all… Booleans are fine, it’s just that later you could have problems due to them. However looks like you had already. They need clean mesh so copy your window’s outline, make that a separate 3D object (add faces, extrude) and use as a form to have boolean Difference with. A lot of trouble and heap of modeling, no?
Here’s one simple: