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
This version of the app includes several new features and minor UI improvements. Most notably, users are now able to display their TOTP codes in a different format. We have also built a feature that allows for users to notify us of QR codes that do not work and we now display more information on push notifications allowing for users to better determine whether to accept or reject these notification.