Welcome to the Apple Support Communities!
I understand you are having trouble backing up your iPhone 7 to iCloud. Is this your first attempt to back up your iPhone? The resource below provides some basic troubleshooting steps for this issue.
If iCloud Backup couldn’t be completed or you can’t restore
Another possibility is to try an iTunes backup on a computer so that you have at least one backup of your valuable data.
How to back up your iPhone, iPad, and iPod touch
Take care.