.kv的结构看起来不正确，例如，观察到有很多根会导致另一个问题，因此如果您想要告诉您问题原因的答案，您应该改进缩进.

The structure of your .kv does not look correct, for example it is observed that there are many roots that would cause you another problem, so if you want a response that tells you the reason for your problem you should improve your indentation.

相反，我将向您显示一个正确的.kv，其中根目录为BoxLayout，您的孩子为ID时间为Label的标签:

Instead I'll show you a correct .kv where the root will be the BoxLayout and your child the Label with id time:

clock.kv

<Label>:
    font_name: 'Roboto'
    font_size: 60
    markup: True

BoxLayout:
    orientation: 'vertical'

    Label:
        id: time
        text: '[b]00[/b]:00:00'

main.py

from kivy.app import App
from kivy.clock import Clock
from kivy.core.text import LabelBase
from kivy.core.window import Window
from kivy.utils import get_color_from_hex
from time import strftime


class ClockApp(App):
    def on_start(self):
        Clock.schedule_interval(self.update, 0)

    def update(self, *args):
        self.root.ids.time.text = strftime('[b]%H[/b]:%M:%S')

if __name__ == '__main__':
    Window.clearcolor = get_color_from_hex('#101216')
    LabelBase.register(name='Roboto', fn_regular='Roboto-Thin.ttf', fn_bold='Roboto-Medium.ttf')
    ClockApp().run()