Dear 林晉安,
Thank you for the feedback. We just released Authenticator v.2.3.1 which includes a fix for enabling Backup, hopefully that fixes the issue you mention, let us know!
Regards,
LastPass
The latest version of the app includes minor bug fixes and UI improvements which include support for large screen devices. We have also removed support for Android 9 as it has reached end of life and is no longer being actively updated with the latest security enhancements.