function takes exactly 2 arguments (1 given)

[m12lrpv]

Admittedly i'm trying to run it on v19
OS: Windows 10 (10.0)
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.19.22522 (Git)
Build type: Release
Branch: master
Hash: d8e476ef428017900dfe0f9359ba448503c216f9
Python version: 3.8.5
Qt version: 5.12.6
Coin version: 4.0.0
OCC version: 7.4.0
Locale: English/Australia (en_AU)

20:23:19  Running the Python command 'CreateInvoluteGear' failed:
Traceback (most recent call last):
  File "C:\Users\Administrator\AppData\Roaming\FreeCAD\Mod\freecad.gears-master\freecad\gears\commands.py", line 43, in Activated
    Gui.doCommandGui("freecad.gears.commands.{}.create()".format(
  File "<string>", line 1, in <module>
  File "C:\Users\Administrator\AppData\Roaming\FreeCAD\Mod\freecad.gears-master\freecad\gears\commands.py", line 61, in create
    cls.GEAR_FUNCTION(obj)
  File "C:\Users\Administrator\AppData\Roaming\FreeCAD\Mod\freecad.gears-master\freecad\gears\features.py", line 127, in __init__
    super(InvoluteGear, self).__init__(obj)
  File "C:\Users\Administrator\AppData\Roaming\FreeCAD\Mod\freecad.gears-master\freecad\gears\features.py", line 88, in __init__
    self.make_attachable(obj)
  File "C:\Users\Administrator\AppData\Roaming\FreeCAD\Mod\freecad.gears-master\freecad\gears\features.py", line 95, in make_attachable
    obj.addExtension('Part::AttachExtensionPython')

function takes exactly 2 arguments (1 given)
Running the Python command 'CreateInternalInvoluteGear' failed:
Traceback (most recent call last):
  File "C:\Users\Administrator\AppData\Roaming\FreeCAD\Mod\freecad.gears-master\freecad\gears\commands.py", line 43, in Activated
    Gui.doCommandGui("freecad.gears.commands.{}.create()".format(
  File "<string>", line 1, in <module>
  File "C:\Users\Administrator\AppData\Roaming\FreeCAD\Mod\freecad.gears-master\freecad\gears\commands.py", line 61, in create
    cls.GEAR_FUNCTION(obj)
  File "C:\Users\Administrator\AppData\Roaming\FreeCAD\Mod\freecad.gears-master\freecad\gears\features.py", line 312, in __init__
    super(InternalInvoluteGear, self).__init__(obj)
  File "C:\Users\Administrator\AppData\Roaming\FreeCAD\Mod\freecad.gears-master\freecad\gears\features.py", line 88, in __init__
    self.make_attachable(obj)
  File "C:\Users\Administrator\AppData\Roaming\FreeCAD\Mod\freecad.gears-master\freecad\gears\features.py", line 95, in make_attachable
    obj.addExtension('Part::AttachExtensionPython')

function takes exactly 2 arguments (1 given)Running the Python command 'CreateInvoluteRack' failed:
Traceback (most recent call last):
  File "C:\Users\Administrator\AppData\Roaming\FreeCAD\Mod\freecad.gears-master\freecad\gears\commands.py", line 43, in Activated
    Gui.doCommandGui("freecad.gears.commands.{}.create()".format(
  File "<string>", line 1, in <module>
  File "C:\Users\Administrator\AppData\Roaming\FreeCAD\Mod\freecad.gears-master\freecad\gears\commands.py", line 61, in create
    cls.GEAR_FUNCTION(obj)
  File "C:\Users\Administrator\AppData\Roaming\FreeCAD\Mod\freecad.gears-master\freecad\gears\features.py", line 487, in __init__
    super(InvoluteGearRack, self).__init__(obj)
  File "C:\Users\Administrator\AppData\Roaming\FreeCAD\Mod\freecad.gears-master\freecad\gears\features.py", line 88, in __init__
    self.make_attachable(obj)
  File "C:\Users\Administrator\AppData\Roaming\FreeCAD\Mod\freecad.gears-master\freecad\gears\features.py", line 95, in make_attachable
    obj.addExtension('Part::AttachExtensionPython')

function takes exactly 2 arguments (1 given)Running the Python command 'CreateCycloidGear' failed:
Traceback (most recent call last):
  File "C:\Users\Administrator\AppData\Roaming\FreeCAD\Mod\freecad.gears-master\freecad\gears\commands.py", line 43, in Activated
    Gui.doCommandGui("freecad.gears.commands.{}.create()".format(
  File "<string>", line 1, in <module>
  File "C:\Users\Administrator\AppData\Roaming\FreeCAD\Mod\freecad.gears-master\freecad\gears\commands.py", line 61, in create
    cls.GEAR_FUNCTION(obj)
  File "C:\Users\Administrator\AppData\Roaming\FreeCAD\Mod\freecad.gears-master\freecad\gears\features.py", line 947, in __init__
    super(CycloidGear, self).__init__(obj)
  File "C:\Users\Administrator\AppData\Roaming\FreeCAD\Mod\freecad.gears-master\freecad\gears\features.py", line 88, in __init__
    self.make_attachable(obj)
  File "C:\Users\Administrator\AppData\Roaming\FreeCAD\Mod\freecad.gears-master\freecad\gears\features.py", line 95, in make_attachable
    obj.addExtension('Part::AttachExtensionPython')

function takes exactly 2 arguments (1 given)Running the Python command 'CreateCycloidRack' failed:
Traceback (most recent call last):
  File "C:\Users\Administrator\AppData\Roaming\FreeCAD\Mod\freecad.gears-master\freecad\gears\commands.py", line 43, in Activated
    Gui.doCommandGui("freecad.gears.commands.{}.create()".format(
  File "<string>", line 1, in <module>
  File "C:\Users\Administrator\AppData\Roaming\FreeCAD\Mod\freecad.gears-master\freecad\gears\commands.py", line 61, in create
    cls.GEAR_FUNCTION(obj)
  File "C:\Users\Administrator\AppData\Roaming\FreeCAD\Mod\freecad.gears-master\freecad\gears\features.py", line 672, in __init__
    super(CycloidGearRack, self).__init__(obj)
  File "C:\Users\Administrator\AppData\Roaming\FreeCAD\Mod\freecad.gears-master\freecad\gears\features.py", line 88, in __init__
    self.make_attachable(obj)
  File "C:\Users\Administrator\AppData\Roaming\FreeCAD\Mod\freecad.gears-master\freecad\gears\features.py", line 95, in make_attachable
    obj.addExtension('Part::AttachExtensionPython')

function takes exactly 2 arguments (1 given)Running the Python command 'CreateCrownGear' failed:
Traceback (most recent call last):
  File "C:\Users\Administrator\AppData\Roaming\FreeCAD\Mod\freecad.gears-master\freecad\gears\commands.py", line 43, in Activated
    Gui.doCommandGui("freecad.gears.commands.{}.create()".format(
  File "<string>", line 1, in <module>
  File "C:\Users\Administrator\AppData\Roaming\FreeCAD\Mod\freecad.gears-master\freecad\gears\commands.py", line 61, in create
    cls.GEAR_FUNCTION(obj)
  File "C:\Users\Administrator\AppData\Roaming\FreeCAD\Mod\freecad.gears-master\freecad\gears\features.py", line 834, in __init__
    super(CrownGear, self).__init__(obj)
  File "C:\Users\Administrator\AppData\Roaming\FreeCAD\Mod\freecad.gears-master\freecad\gears\features.py", line 88, in __init__
    self.make_attachable(obj)
  File "C:\Users\Administrator\AppData\Roaming\FreeCAD\Mod\freecad.gears-master\freecad\gears\features.py", line 95, in make_attachable
    obj.addExtension('Part::AttachExtensionPython')

function takes exactly 2 arguments (1 given)

[m12lrpv]

Solution.

Find features.py in your personal mods folder (if you were able to install it. I had to pull the repo)
C:\Users\{YourUserName}\AppData\Roaming\FreeCAD\Mod\freecad.gears\freecad\gears\features.py
change if int(App.Version()[1]) >= 19: to be if int(App.Version()[1]) >= 20:

    def make_attachable(self, obj):
        # Needed to make this object "attachable",
        # aka able to attach parameterically to other objects
        # cf. https://wiki.freecadweb.org/Scripted_objects_with_attachment
        print(App.Version())
        if int(App.Version()[1]) >= 20:
            obj.addExtension('Part::AttachExtensionPython')
        else:
            obj.addExtension('Part::AttachExtensionPython', obj)
        # unveil the "Placement" property, which seems hidden by default in PartDesign
        obj.setEditorMode('Placement', 0) #non-readonly non-hidden