It's been created by extruding the main body to full width, then doing an extrude cut to leave the lip, which means you then can't select the face of the lip to do an extrude join all the way to the opposite side. All you can select is the original non-cut surface, or the new recessed cut surface.
So the easiest solution with the existing model, is as per my previous post, and just create a new sketch on the opposite face, and extrude as needed.
Just realised the problem.
You've created two sketches. The first sketch (Sketch2) is blank, and the second sketch (Sketch3) has it's plane referenced of the first sketch, so due to how the actually extruded sketch plane has been referenced, it blocks you from selecting the narrow face.
If you delete Sketch2, then right click Sketch3 in the menu system (see screenshot), select Redefine Sketch Plane, then click on the relevant extruded surface, it will then let you select the lip so you can do an Extrude (select Join, as it'll likely default to cut when you try pushing the extrude through the body).
That's assuming you want to extend the lip to the rear. If you want to cut the recess, then you'll still need draw a new circle on the rear surface.