Unfortunately your EXTG and PROP groups are very very complex, and have many faces and edges that are almost coincident.
The native Solids Subtract tool will let you take one from the other, BUT the results are made non-solid, with 0.0 volume.
Because CutNfill relies on these tools, that is why you can't get meaningful results.
SketchUp's built-in tolerance is 1/1000" and any edges that would be shorter that that are ignored, as if their end/start points are coincidental, but the tiny gap is not 'healed'. Consequently, any faces relying on such missing tiny edges will fail to form, and the result is a non-solid group with 0.0 volume.
So you need to make your modeling much simpler and ensure that this tiny geometry issue is sidestepped.
In real life cut and fill is not an exact science on site, so any approximation does not need to be perfect.