Operation failed because resulting body wouldn't be valid... hollow body import STEP file.

I have downloaded and imported a STEP file to edit, however the body is hollow. In the screenshots I am including, the “original” body from the same source is solid, and the “lightweight” edited model is hollow/shell. When I try to adjust dimensions, or any feature, I end up with the error message bar in the title. I have messed with this thing for several days now trying to make the interior of this body solid somehow. Just can’t figure it out.

The intentional difference between the two files, the one on the left is purposely minimalist to reduce weight. The issue is adding a fillet/chamfer, adjusting
dimensions, etc. You can see where I’d want the solid, the right image shows the section view with the striped tan, and the left is just a shell that I cannot seem to fill.