i selected two solids to do the subtraction, and after that, the resulting object is no longer a solid, when I look carefully, I saw there is an opening in an unrelated area
I checked the file and the group instance called difference shows as a solid as well as the other object. Can you provide more description of the file and what was exactly done.
Did you try running the plugin Solid Inspector2 , very helpful for tracking solid issues.
Also, the objects are small , 3 13/16" square and sometimes SketchUp doesnt work well with small geometry. You could try the “Dave Method”, using components with a scaled up copy. It usually helps.
https://forums.sketchup.com/t/the-dave-method/49720