Runtime error while adding a new field to the existing feature layer using Python

Question

I have been trying to add a new field to one of my feature layers. And I keep getting an error and I can't figure out what is going wrong.

I am attaching my code below for reference:

When I changed the spatial reference of the 'tracts10_area', it has created a new feature layer called 'tracts10_area_1' in the mentioned workspace.

I have been trying to add a new field called "Populationdesnity" to my existing layer called "tracts10_area_1". And this is the error that is shown:

input_features = ["tracts10_area"]
out_workspace = r"S:\Grad School\FALL 2020\CEE 424 - GIS for Civil Engineers\Homeworks\HW6 PythonEdition\censusGDB\censusGDB\KingCounty_GDB_census.gdb"
out_coordinate_system = arcpy.SpatialReference(4326)
arcpy.BatchProject_management(input_features, out_workspace, out_coordinate_system)
<Result 'S:\\Grad School\\FALL 2020\\CEE 424 - GIS for Civil Engineers\\Homeworks\\HW6 Python Edition\\censusGDB\\censusGDB\\KingCounty_GDB_census.gdb'>
lyrfile = arcpy.mapping.Layer(r"S:\Grad School\FALL 2020\CEE 424 - GIS for Civil Engineers\Homeworks\HW6 Python Edition\censusGDB\censusGDB\KingCounty_GDB_census.gdb\tracts10_area_1")
 arcpy.AddField_management("tracts10_area_1","Populationdensity","LONG",6,"","","pop_den","NULLABLE","NON_REQUIRED")

Runtime error Traceback (most recent call last):
File "<string>", line 1, in <module> File "c:\program files (x86)\arcgis\desktop10.6\arcpy\arcpy\management.py",
line 3424, in AddField raise e ExecuteError: The value cannot be a joined table 
 ERROR 000840: The value is not a Raster Layer. 
 ERROR 000840: The value is not a Raster Catalog Layer. 
 ERROR 000840: The value is not a Mosaic Layer.

ERROR message along with the code

Answer 1

The simplest solution would be something like:

myFC = r"S:\Grad School\FALL 2020\CEE 424 - GIS for Civil Engineers\Homeworks\HW6 Python Edition\censusGDB\censusGDB\KingCounty_GDB_census.gdb\tracts10_area_1"
arcpy.AddField_management(myFC,"Populationdensity","LONG",6,"","","pop_den","NULLABLE","NON_REQUIRED")

This bypasses the need for creating a layer object and directly references the Feature Class.

If you need a layer object consider using MakeFeatureLayer tool then you can reference the layer object with it's name as you have done in your code:

i.e. arcpy.AddField_management("tracts10_area_1","Populationdensity","LONG"...