To clarify: this is packages (in this case Orchid) which use a class name which is already in use (in this case List).
There are only a few packages which still cause this headache, and I believe that these may be blocked from use at some point.
I believe that following the guidance @GavinCrump provided should resolve the list issue and so the subsequent lines of design script would process.
Since you only need the inside face, you could also stop the current code after the exterior face was retrieved, and then define the interior face to be something like:
IntFaceCurves =
wallcurves.Translate(direction,-distance);