Look at the following areas of the SketchUp interface for the Solid Tools: Toolbar for solids the menu of tools (Select Tools > Outer Shell or Solid Tools, then choose the other tools from a submenu.) Palette of tools (macOS)
You may make new shapes with SketchUp's Solid tools by combining or cutting one shape with another, making it simple to build an exterior shell or joinery.
You haven't bothered to make it a group or component, which is why it isn't solid. (Funny enough, that was my guess in my first post.) Simply choose the geometry and convert it to a group or component.