My iPhone is full, so I’m trying to back it up.
I’ve gone to “Settings” => iCloud => Backup and click “Backup Now”.
The little wheel starts going, and it says “Backing up … / Estimating time remaining …”
Then it stops and says “Last backup could not be completed.”
Any ideas what’s up? Why won’t it do the back-up? 
Are you out of space in your icloud account? I tried backing up to icloud ONCE. When I found out the amount of space they give me for free on icloud wasn’t even enough to back up my phone I said bye to icloud and never use it for anything.
I’ve got 963 GB in iCloud, so I don’t think that’s the problem.
Instead of using wifi to do a backup, have you tried connecting to a computer and backing up via iTunes? That’s what works for me.
I’m travelling at the moment, but will try that when I get home. Thanks!
That’s most likely the problem- iCloud backup doesn’t work over cellular. And if you were connected to someone else’s wifi, your connection might have been throttled to the point where it was much too slow to do a backup. And anyway, you probably don’t want to be backing up your data over someone else’s wifi.