I am currently using Revit and for my class project I decided to create the Eiffel tower. I traced the Eiffel tower using auto cad and imported it to Revit, I then extruded the outline of the Eiffel which makes a base of where my structure is. To be able to make the void and spaces in between the Eiffel, I clicked on void extrusion and trace the tower. When I click finish it pops up a warning and I am unable to move to the next step.