Hi,
Corner windows can be a real pain - but they are possible.
The main problem, is that you don't appear to have a void extrusion set up for the return part of the window. The main window opening part is defined in your model with an 'Opening Cut'. Revit doesn't like combining Opening Cuts and Void Extrusions to cut the host wall - it's either/or (AFAIK it won't let you). You have a void extrusion in the model, but it is linked in with the window frame part of the return, and is not cutting the host wall (it is also too low).
I would delete the 'opening cut' and form a void extrusion that includes both legs of the 'L' - align it with the various reference planes - then use it to cut the host wall.
Martin
