Some customers have reported that logging out of their iCloud account and then signing back in has solved their iCloud backup problem. This method has long been used to resolve account verification difficulties that may be preventing iCloud backups from succeeding. It won't harm to try it again this time and see what happens. Here's how you do it:

1. From the Home screen, choose Settings.
2. Scroll down to iCloud and press it.
3. Sign out by scrolling to the bottom of the screen.
4. Confirm that you wish to remove all of your preferences. You will be logged out of your iCloud account and routed to an iCloud sign-in screen after doing so.
5. Enter your iCloud account and password to log back in.
6. To sign in entirely, follow the onscreen steps.
7. Once you've regained access, try backing up your iPhone to iCloud once more.

If you sign out of iCloud, you will not lose all of your data. While you may receive a notification stating that you will be removing or deleting data from your iPhone, you will not lose anything since all of your information will be saved in iCloud Drive for safety. You will still be able to access your files as a result of this.

All of your data will be instantly redownloaded to your device once you sign back in with your iPhone.