If an alert says that there's not enough iCloud storage In iOS or iPadOS, go to Settings > [your name]. Tap iCloud. At the top, you can check how much iCloud storage you've used. Tap Manage Account Storage or Manage Storage. Tap Backups, then tap the device you're using. Under Last Backup and Backup Size, you can check Next Backup Size.

To check and see if storage is the problem: Open the Settings app, and tap your profile at the top of the page. Tap iCloud. You'll see your iCloud storage summary at the top of the screen. If your storage is maxed out, your iPhone iCloud backup won't work. Read on to learn how to clear some space or buy more iCloud storage.