Back up

What do you see when you go to Settings/[your name]/iCloud/iCloud Backup and tap Back Up Now?

Or are you trying to back up using your computer? What kind of computer, what operating system, and what steps are you following?

You need to “rent” additional iCloud storage. Go to Settings/[your name]/iCloud/Manage Storage/Change Storage Plan and update to 50 GB for 99 cents a month.