Elinor, I see some issues that are preventing a shell. Primarily, the radius of some of your boundary surface sketches is too small to then offset the surfaces inside to create the inner surface of a shell. Check Sketches 29 and 33 particularly, where it seems you've got a Fit Spline feature (or similar) with a tiny radius in some areas.
If you're using Fit Spline with these, make sure the settings are generating a spline that really fits your underlying arc geometry. To me, it appears the Fit Spline is making a smaller radius than is necessary, which won't allow the inside surface to be created (0.080" offset inward) for your shell. The minimum radius for all sketch geometry in this case should be slightly greater than your wall thickness, so you'll want to also check your other boundary surfaces to make sure this is correct.
I need to get back onto another project for now, but you can contact me here, if necessary: