Hi there! The Cisco community is not "support" in that internal support staff actively monitor community posts or that there is an SLA for a response from support staff here. I am not even part of Duo's support team, I am just someone who knows a lot about our products and likes to answer questions.
What error message do your users receive when trying to enable backup to Google Drive? Does it help if they toggle Duo Restore off and on again in the app?
Are there any controls being applied by the org on Google Drive? For example, are the phones enrolled in an MDM or managed by a work profile that might restrict Google Drive access or Duo Mobile app registration/permissions in Google Drive? This is a known issue with Intune-managed devices, and possible other MDMs.