Split features script returning Python ERROR 000840: The value is not a Feature Layer

I am trying to split a feature layer by rows. I've previously some time ago been able to use the below script, however now when I use it, it returns:

Python ERROR 000840: The value is not a Feature Layer

import arcpy

outputNum = 5000000
outputFCName = "Flows" 

def listSplit(myList, n):
    for i in xrange(0, len(myList), n):
        yield myList[i:i + n]

arcpy.env.workspace = r"C:\...\Database.gdb" #The dots are replaced with a proper path
lyr = arcpy.mapping.Layer("Layername") 
arcpy.SelectLayerByAttribute_management(lyr, "CLEAR_SELECTION")
fList = list()

with arcpy.da.SearchCursor(lyr, "OID@") as cursor:
    for row in cursor:
        fList.append(row[0])

listGroup = listSplit(fList, outputNum)

for x in listGroup:
    lyr.setSelectionSet("NEW", x)
    arcpy.CopyFeatures_management(lyr, arcpy.CreateUniqueName(outputFCName))

The reason for splitting is that I have a dataset with 24.000.000 rows, which I would prefer in smaller parts.