Cause of ERROR 000732: Input Layer: Dataset plan does not exist or is not supported?

Question

I have several mxds (located in C:\Project ) that have 250 layers each. All the layers as the same name- "plan" and they all in group layers like that (added picture for example):

I try to change all symbology layers of "plan" to the same symbology of layer "landuse":

All "plan" layers located one folder that divided to a lot of sub folders and sub-sub folders and their names (in the folders) are "kavim_kchulim". i try this code:

import arcpy,os,sys,string
import arcpy.mapping
from arcpy import env

env.workspace = r"C:\Project"
symbologyLayer = r"C:\Project\layers\landuse.lyr"
for mxdname in arcpy.ListFiles("*.mxd"):
    print mxdname 
    mxd = arcpy.mapping.MapDocument(r"C:\Project\\" + mxdname)
    dfList = arcpy.mapping.ListDataFrames(mxd, "*")
    for df in dfList:
        print df.name 
        for lyr in arcpy.mapping.ListLayers(mxd, "", df):
            if lyr.name == "plan":
                arcpy.ApplySymbologyFromLayer_management("plan", symbologyLayer)
                print 'changed'     
    mxd.save()
del mxd

but get an error:

>>> 
Project.mxd
Layers

Traceback (most recent call last):
  File "C:\Users\yaron.KAYAMOT\Desktop\python.py", line 16, in <module>
    arcpy.ApplySymbologyFromLayer_management("plan", symbologyLayer)
  File "C:\Program Files (x86)\ArcGIS\Desktop10.3\ArcPy\arcpy\management.py", line 6470, in ApplySymbologyFromLayer
raise e
ExecuteError: Failed to execute. Parameters are not valid.
ERROR 000732: Input Layer: Dataset plan does not exist or is not supported
Failed to execute (ApplySymbologyFromLayer).

Answer 1

You should refer to the layer (lyr), not to "plan" when applying the symbology:

arcpy.ApplySymbologyFromLayer_management(lyr, symbologyLayer)

"plan" is just a string so arcpy tries to locate it in your predefined workspace.