It might be the same problem I had a few weeks back: Create Walls using Wall.ByFace from a DirectShape Mass
In which case, for some reason any mass created directly through Dynamo is not a valid mass face for use with API or Wall.ByFace nodes. However, they can still be used within Revit using the built in tools. I don’t know if I am just missing something or if it is a bug within Dynamo. Maybe @jacob.small can tell?