Hello there @dmaraujo!
It’s because your building’s mesh is too simple (only 6 faces), in its place I would input the building’s brep in order to set the simulation’s grid size. If you hover your mouse pointer over the _geometry input you will read this tooltip:
Rhino Breps and/or Rhino Meshes for which incident radiation analysis
will be conducted. If Breps are input, they will be subdivided using
the _grid_size to yeild individual points at which analysis will
occur. If a Mesh is input, radiation analysis analysis will be
performed for each face of this mesh instead of subdividing it.
However, if you were to continue working with meshes you should further subdivide and refine the mesh geometry to get a finer resolution. Hope this helps.