I'm modifying an existing model (cut a hole), and for some reason I can't sketch on the faces. I see it's a solid body in the model I'm using, so I got the original, and I can't sketch on that either. Confused, I made a simple lofted model myself, and I was able to select and sketch on any face. The goal is the hole in the second image, which I did on a plane instead of the face. Of course, I can't do a coincident mate to the face with the connector that's going in the hole because it's at an angle.