How to grant write access to external SDcard using ADB in Android tv?
Android Marshmellow (API 23) or above you need to grant runtime permission. This can be done through an adb shell with the command:

adb shell pm grant com.your.app.package android.permission.WRITE_EXTERNAL_STORAGE


If you ever need to revoke the permission the use this command:

adb shell pm grant com.your.app.package android.permission.READ_EXTERNAL_STORAGE