SketchUp's Solid tools work only on SketchUp solids. In SketchUp, a solid is any 3d model (component or group) that has a finite closed volume. A SketchUp solid cannot have any leaks (missing faces or faces that do not meet at an edge). You can select a component or group and visit the Entity Info dialog box to see if the selection is a solid. If there is a volume listed, the selection is a SketchUp solid. If there is no volume listed, the selection is not a solid and likely has a leak.