Having problem cutting a hole

Trying to cut a hole, got the first one, but having trouble with second one.

fireplace mantel.skp (503.2 KB)

On the front side (inside) you can erase the face.
On the other side of the wall the edges show as profiles (thick lines), not cutting out a separate inner face area. Redraw one of the edges here and the large wall face will split into two. (wall v.s. window area). The edges become thin. Erase the inner face now.

Take a look at this x-ray view of your model. You have drawn the back outline and its edges through the wall out of line with the front outline! Aside from what @Wo3Dan has pointed out, the misalignment is going to cause issues with stray edges. This raises the question: why not draw the outline on just the front face and pushpull it through?

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To throw another twist. The inside window has a ledge. It’s even with the wall, so the window actually fist on the backside.

Do I need to pick a different 3D model

You just need to decide what part of the window defines the hole and make the hole that size and shape. As is, the faces you’ve made on the inside and outside are not consistent so the answer isn’t clear. Think clearly about what you did differently on the second versus the first window.