“Description” is a Type parameter so you need to switch out the nodes Categories (Walls) > All Elements of Categories to Element Types ( Wall Types ) > All Elements of Type. Right now you’re collecting wall instances so they will not have the Description parameter.
You’ll also want to ensure that your list of collected wall type elements follows the order of the wall types from your Excel file. If a column of your Excel schedule contains the Wall Type name (exactly as it is in Revit), you could eliminate the All Elements of Type part of your workflow and use the OOTB WallType.ByName node and then your wall type elements will already be in order of your Excel data
A final suggestion would be to eliminate the hard returns in your Description (Assembly) output, as the OOTB parameter “Description” is not a multi-line text parameter
Hope this helps