"Last backup could not be completed" when "Last backup: Never"
From the date of your post, you were using iOS 9 at the time. I experienced this issue myself with backups in iOS 9. Forcing another iCloud backup would eventually allow it to complete, but over time I had to deal with runaway backup sizes (growing up to upwards of 5 GiBs after a few days).

As far as I can tell, this appears to be resolved at last in iOS 10. I would recommend performing a backup in iTunes, then upgrading before trying another iCloud backup.