hey guys im new in kivy so excuse me if it was a silly question, i have an error in kivymd that says: Unknown class i have the library(kivymd) and kivy library

Screen:
    BoxLayout:
        orientation: 'vertical'
        MDToolbar:
            title: 'Demo Application'
            left_action_items: [["menu", lambda x: app.navigation_draw()]]
            right_action_items: [["dots-vertical", lambda x: app.callback()], ["clock", lambda x: app.callback_2()]]
            elevation:5
            
        MDLabel:
            text: 'hello world'
            halign: 'center'

and the python is

from kivymd.app import MDApp
from kivy.lang.builder import Builder
from kivy.core.window import Window
from kivymd.theming import ThemeManager
import kivy


class UltimateApp(MDApp):
    theme_cls = ThemeManager()
    def build(self):
        
        return Builder.load_file("main.kv")
        

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

i tried to install the kivymd library and it said i already installed it

and i searched in stack overflow and i didnt found anything