"No solids" mystery

SketchUp
1

I’ve been working for a while on this, and all of the sudden I can no longer use the outer shell tool because it tells me some things are not solids. I’ve investigated every shape and carefully build them, I can’t seem to understand why this happened and why at that point of working on the file. I even tried copying groups that were not a problem before to a new file, but now everything seems to have a problem with the solid. I’ve attached the file below, any help would be much appreciated.

Roodtop 2.stl (151.7 KB)

2

The .stl isn’t much use for working out the solid issue, attach the .skp

3

Oh sorry, here’s the SKP: Roodtop 2.skp (738.1 KB)

4

In order for a component or group to be considered a solid, it must contain only raw geometry (edges and faces) and every edge must be share by exactly two faces. No more and no less. So no nested groups/components. No stray edges, no holes in surfaces, and no internal faces.