I'm stuck with kivy's mapmarkerpopup function, can anyone help me?

Question

At the moment I'm having problems displaying a marker to insert on the map although the code from IDE does not give errors of any kind, it displays the map but not the marker.

This is the python code:

from kivymd.app import MDApp
from kivy.lang import Builder
from kivy.garden.mapview import MapView
import mysql.connector

#collegamento al database in locale
db = mysql.connector.connect(
    host = "localhost",
    user = "root",
    password = ""
    )
class MapViewApp(MDApp):
    def build(self):
        self.screen = Builder.load_file("main.kv")
        return self.screen

MapViewApp().run()

this is the 'main.kv':

#:import MapSource kivy_garden.mapview.MapSource

MDRelativeLayout:
        MapView:
                lat: 44.29
                lon: 11.20
                zoom: 10
                double_tap_zoom: True


                MapMarkerPopup:
                    #   source: "simbolo da usare come popup"
                        lat: 44.29
                        lon: 11.20

Can anyone help me understand or know where I'm wrong?

I have tried several times to search for some solution online, but the only thing is that probably given that the version is 2.3.0 it might not work. I checked if I installed everything