- On your Home screen, go to Settings.
- Scroll to iCloud, then touch it.
- To sign out of your iCloud account, tap Sign Out.
- Confirm that you wish to remove all settings if asked. You will be logged out of your iCloud account and forwarded to an iCloud sign-in screen if you do so.
- Enter your iCloud login credentials, then complete the sign-in process by following the onscreen prompts.
Try backing up your iPhone 7 to iCloud again after enrolling into your iCloud account to see if you are now allowed to do so.
If your device still won't back up to iCloud, your last option is to restore your iPhone 7 in DFU mode. It's the most thorough sort of iOS restore, erasing both your device's hardware and software settings. As a result, any flaws creating sophisticated software difficulties on the device may be removed as well. If you have a computer with the most recent iTunes software installed, you might try restoring your iPhone 7 in DFU mode.