Backups will only succeed when your WP website is online, visible and publicly accessible.
This might be obvious to old-timers, but it could be worth pointing out that backups using Online Backup For WordPress will fail if they are attempted while your WP website is online but in "Maintenance Mode" via a plugin such as "WP Maintenance Mode". This was not so obvious to me (doh!) while I was building my website online (in Maintenance Mode) and trying to back it up between builds.
I know better now!!