I am attempting to use the Complex By Elements tool to create horizontal geometry. When I try to select my first element, I get the error message, "Element not valid for tool".
The most likely reason for this error message is that you are trying to create complex horizontal geometry from 3D elements. Horizontal geometry should be based on 2D elements. You can use the tool, Plan By 3D Element, to create a 2D element from a 3D Drawing element. The original 3D element will be used as the active profile for the newly created 2D element.