Can't close a 3D surface

Hi everyone!
So i was trying to close up this 3D model that i made, but somehow i can’t manage to do it
can anyone help?bingkai.skp (140.1 KB)

Use a face to cut the shape.

Also turn your faces around so the white are the outside and blue/gray inside, this is how software knows which is in and out. Note the Copy/Paste options can be found in the edit menu, I have them in the context menu for display only.