Unknown class in Kivy

Сделал тестовую программу на Xubuntu18.04, но получил ошибку.когда запускал на Windows все работало.
Code:

from kivy.app import App
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.graphics import Rectangle
from kivy.uix.widget import Widget
from kivy.uix.image import Image
from kivy.uix.button import Button
from kivy.clock import Clock
from kivy.properties import NumericProperty,ColorProperty,ReferenceListProperty,BoundedNumericProperty, ListProperty,StringProperty
from kivy.core.window import Window



from kivy.lang.builder import Builder

Builder.load_string('''
#:import Factory kivy.factory.Factory
Manager:
    Menu:
        name:'menu'
    Game:
        name:'game'

<Menu>:
    BoxLayout:
        orientation: 'vertical'
            
        BoxLayout:
            orientation: 'vertical'
            padding: '50dp'
            spacing: '20dp'
            
            Button:
                text: 'GO'
                on_release:
                    app.root.transition.direction = 'down'
                    app.root.current = 'game'
                                        
            Button:
                text: 'Выйти'
                on_release:
                    app.stop()


<Game>:
    BoxLayout:
        orientation: 'vertical'
        canvas:
            Color:
                rgba: 0,1,0,1
            Rectangle:
                size: self.size
                pos: self.pos

''')

class Manager(ScreenManager):
    pass
class Menu(Screen):
    def on_enter(self, *args):
        print('Menu')
        
class Game(Screen):
    def on_enter(self, *args):
        print('hello world')
        
class Main(App):
    pass

if __name__ == '__main__':
    Main().run()

Error:

[INFO   ] [Logger      ] Record log in /home/xubuntu/.kivy/logs/kivy_20-08-10_52.txt
[INFO   ] [Kivy        ] v1.11.1
[INFO   ] [Kivy        ] Installed at "/usr/local/lib/python3.6/dist-packages/kivy/__init__.py"
[INFO   ] [Python      ] v3.6.9 (default, Nov  7 2019, 10:44:02) 
[GCC 8.3.0]
[INFO   ] [Python      ] Interpreter at "/usr/bin/python3"
[INFO   ] [Factory     ] 184 symbols loaded
[INFO   ] [Image       ] Providers: img_tex, img_dds, img_sdl2, img_pil, img_gif (img_ffpyplayer ignored)
[INFO   ] [Text        ] Provider: sdl2
[INFO   ] [Window      ] Provider: sdl2(['window_egl_rpi'] ignored)
[INFO   ] [GL          ] Using the "OpenGL" graphics system
[INFO   ] [GL          ] Backend used <sdl2>
[INFO   ] [GL          ] OpenGL version <b'4.3 (Compatibility Profile) Mesa 19.2.8'>
[INFO   ] [GL          ] OpenGL vendor <b'nouveau'>
[INFO   ] [GL          ] OpenGL renderer <b'NVE7'>
[INFO   ] [GL          ] OpenGL parsed version: 4, 3
[INFO   ] [GL          ] Shading version <b'4.30'>
[INFO   ] [GL          ] Texture max size <16384>
[INFO   ] [GL          ] Texture max units <32>
[INFO   ] [Window      ] auto add sdl2 input provider
[INFO   ] [Window      ] virtual keyboard not allowed, single mode, not docked
 Traceback (most recent call last):
   File "main.py", line 54, in <module>
     ''')
   File "/usr/local/lib/python3.6/dist-packages/kivy/lang/builder.py", line 399, in load_string
     widget = Factory.get(parser.root.name)(__no_builder=True)
   File "/usr/local/lib/python3.6/dist-packages/kivy/factory.py", line 131, in __getattr__
     raise FactoryException('Unknown class <%s>' % name)
 kivy.factory.FactoryException: Unknown class <Manager>