You are currently working in the Custom Node editor environment (.dyf), indicated by the yellow background.
In this environment you can’t run scripts, only create Custom Nodes to use in scripts.
What you could do is copy-paste the Python node itself into a .dyn-workspace and see if running the code there (on a single item, not a list) returns any errors.
Here is a FilteredElementCollector to grab the walls and an if statement to grab ones of the type you are looking for.
import clr
clr.AddReference("RevitServices")
import RevitServices
from RevitServices.Persistence import DocumentManager
doc = DocumentManager.Instance.CurrentDBDocument
clr.AddReference("RevitNodes")
import Revit
clr.ImportExtensions(Revit.Elements)
clr.ImportExtensions(Revit.GeometryConversion)
clr.AddReference("RevitAPI")
from Autodesk.Revit.DB import *
#collect all walls in the project...
walls = FilteredElementCollector(doc).OfClass(Wall).WhereElementIsNotElementType().ToElements()
#Empty Lists
list = []
#code
for w in walls:
if w.WallType == ViewType.userWallType:
list.append(w)
else:
pass
#output
OUT = list
I would keep this website handy when working with the FilteredElementCollector, I think your references were coming in wrong. Lot of good stuff on this site to get it to do exactly what you want it to.