The method needs Revit Faces, whereas you are feeding Dynamo Faces.
You can get Revit faces from Dynamo faces like this:
Python:
import clr
clr.AddReference('ProtoGeometry')
from Autodesk.DesignScript.Geometry import *
#Import the Revit API
clr.AddReference('RevitAPI')
import Autodesk
from Autodesk.Revit.DB import *
#Import DocumentManager and TransactionManager
clr.AddReference('RevitServices')
import RevitServices
from RevitServices.Persistence import DocumentManager
#Import ToDSType(bool) extensions method
clr.AddReference('RevitNodes')
import Revit
clr.ImportExtensions(Revit.Elements)
clr.ImportExtensions(Revit.GeometryConversion)
#Reference the active Document and application
doc = DocumentManager.Instance.CurrentDBDocument
faces = IN[0]
output = []
for i in faces:
elemRef = i.Tags.LookupTag('RevitFaceReference')
elem = doc.GetElement(elemRef)
output.append(elem.GetGeometryObjectFromReference(elemRef))
#Assign your output to the OUT variable.
OUT = output