On Android, Plugin uses Saved Games feature of Play Games Services. Internally, Cloud services depend on Google play login for user authentication. For this reason it needs Play Services Application Id.
If you are NOT planning to use Game Services along with Cloud Services, you still need to fill the Play Services Application Id entry in Game Services settings (and can disable it).
Check here (Android Properties tab) on how to get Play Services Application Id.
As cloud services on Android uses google play services(Game Services) internally, you need to make sure the required authentication is handled. You can check here on how to add required credentials for Cloud Services to authenticate successfully.