When I create the mesh, the solid inside the mesh doesnt appear. Why is happening this?
your base mesh box that defines the flow domain around your geometry for an external aerodynamics mesh does not contain the geometry. See the image below. You need to adjust the coordinates of the minimum and maximum point of the box to fit to your model, then you are fine with your setup!
But actually when I place the geometry in the mesh it still doesnt appear.
Indeed - the second setup looks good. It might be due to the fact that the STL is not a manifold model, see upload log:
From what CAD system did you export the model? In there, is it a true solid body / normal part or a surface model? With a solid body, you should be save.