Why Can't I back up my icloud?
I just got a new phone and I need to back it up to my icloud. It's telling me that I am unable to because it's full but I don't believe it is. It's still have some space left.
See the "See how much iCloud storage you're using", "Make more space available in iCloud", and "Manage your iCloud Backup" sections in:
Manage your iCloud storage - Apple Support