After upgrading to latest WPBackItUp version and you see no Available Backups. 


Go to the WPBackitup settings page.  Check Remove Data on  Uninstall and Save. 

Then Go to the plugins page.  First Deactivate WPBackitup, Delete and Re-install the plugin again.  

Note: If you need further assistance please get in touch with WPBackItUp Support and submit a ticket.