Кликер

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.button import Button
from kivy.uix.label import Label
 
class MainApp(App):
    def build(self):
        main_layout = BoxLayout()
        self.label = Label(text = "", size_hint = (.7, .9), pos_hint = {'center_x': 0.5, 'center_y': 0.7})
        btn = Button(text = "Click", size_hint = (.5, .1), pos_hint = {'top_x': 0.5, 'top_y': 0.5})
        btn.bind(on_press=self.on_button_press)
        main_layout.add_widget(self.label)
        main_layout.add_widget(btn)
        return main_layout
 
    def on_button_press(self, instance):
        i = 0
        while i <=20:
            i + 1
            self.label.text = "Clicked " + str(i)
            
 
if __name__ == '__main__':
    app = MainApp()
    app.run()

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.button import Button
from kivy.uix.label import Label
import time
class MainApp(App):
    def build(self):
        main_layout = BoxLayout()
        self.label = Label(text = "", size_hint = (.7, .9), pos_hint = {'center_x': 0.5, 'center_y': 0.7})
        btn = Button(text = "Click", size_hint = (.5, .1), pos_hint = {'top_x': 0.5, 'top_y': 0.5})
        btn.bind(on_press=self.on_button_press)
        main_layout.add_widget(self.label)
        main_layout.add_widget(btn)
        return main_layout
 
    def on_button_press(self, instance):
        i = 0
        while i <=20:
#здесь у тебя была ошибка и цикл получался безконечным
            i += 1 
            self.label.text = "Clicked " + str(i)
 
 
if __name__ == '__main__':
    app = MainApp()
    app.run()