Arthas Wang:
One way is to view the Curtain Wall (CW) in elevation, select it, click the Edit button, make the Scheme Grid visible (it may be helpful to make the other elements not visible), a heavy boundary will appear that can be edited with the Pet Palette similar to a Fill.
There is an older method using the Extract Boundary to Fill command.
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
David