This approach has helped some users who were experiencing the identical problem with their iPhone 6s not being able to backup to iCloud. If you want to give it a try, here's how to do it:

1. Go to the Settings tab.
2. Tap iCloud after scrolling to it.
3. To sign out, use the Sign Out option.
4. Confirm that you want to remove all settings if prompted. You will then be led to an iCloud sign in page after being logged out of your iCloud account.
5. To sign back in, enter your iCloud account credentials.
6. To sign in entirely, follow the onscreen directions.
7. After you've signed in, try backing up your iPhone 6s to iCloud again to see whether it now works.

While you may receive a notice stating that you are about to discard or delete data from your iPhone when you sign out of your iCloud account, you will not lose any data because all of your files will stay in your iCloud Drive. You will, however, be able to access them at any time. When you sign back into iCloud on your iPhone, all of your data will be instantly downloaded.