Q: iOS unable to backup iPhone

Does that happen also when you back it up using iTunes?

Plug in to power and go to Settings > iCloud > Backup > Backup > Backup Now and see whether it shows an error also.

If your iTunes backup couldn't be completed or you can't restore from a backup - Apple Support

There are several working options in this thread

Re: "the last backup could not be completed"

Which go to turn off iCloud and back on to force restart, to fully restore your device

I know it's not preferred, but it's one way of testing the fix until you find the solution and make sure you have all your data safe while doing so. If as many point in that thread and googling the issue, removing the current iCloud backup solves it, at least you have one local copy if anything else fails.

Two simple things to try would be to first power off and then back on the phone, and then try backing up using a different WiFi network.  It is possible that your router, firewall, or ISP is causing a network connectivity issue to Apple's backup servers.  It's simple enough to rule this out by going to a friends house and give it a try from there prior to attempting other solutions.