你的应用需要请求存储权限才能访问它的私有文件夹之外的文件。下面是工作示例。我将os.mkdir更改为os.makedirs，以便能够创建你的路径的父目录。
main.py

from kivy import platform
from kivy.factory import Factory
from kivy.app import App
import os

class TestApp(App):

    def on_start(self):
        if platform == "android":
            from android.permissions import Permission, check_permission, request_permissions
            perms = [Permission.READ_EXTERNAL_STORAGE, Permission.WRITE_EXTERNAL_STORAGE]
            if not all([check_permission(perm) for perm in perms]):
                Factory.MyPermReqPopup().open()

    def popup_close(self):
        from android.permissions import Permission, request_permissions
        request_permissions([Permission.READ_EXTERNAL_STORAGE, Permission.WRITE_EXTERNAL_STORAGE])
        exit()

    def create_folder_click(self):
        if platform == "android":
            from android.storage import primary_external_storage_path
            os.makedirs(str(primary_external_storage_path()) + "/Android/data/org.app.test/folder", exist_ok=True)

TestApp().run()

test.kv

BoxLayout:
    orientation: 'vertical'
    size: root.width, root.height
    Button:
        text: 'create folder'
        on_release: app.create_folder_click()

<MyPermReqPopup@Popup>:
    auto_dismiss: False
    on_dismiss: app.popup_close()
    title: 'Please grant storage permission, then launch app again'
    size_hint: (1, 0.3)
    BoxLayout:
        size: root.width, root.height
        Button:
            text: 'OK'
            on_release: root.dismiss()