Can not create a face on a regular shape object

I’m trying to create a ceiling for an irregular shaped floor plate, using the line tool and tracing the wall lines to fill the face, but it’s not happening. I have attached the .skp file and any advise is appreciated.
Thank you
Rachel
0177_Model2.skp (3.6 MB)

The four points are not coplanar, so you must also create a diagonal, which will create two triangular faces.

Thank you for this.