iCloud Backup not working even though there is enough storage space

See this:

If you can't back up to iCloud or restore from iCloud backup failed - Apple Support

Here's how to find your Next Backup Size:

• On iOS 11 or later and iPadOS: Go to Settings > [your name] > iCloud > Manage Storage > Backups. Tap the device you're using.
• On iOS 10.3: Go to Settings > [your name] > iCloud > iCloud Storage > Manage Storage. Tap the device you're using.

To check how much available storage you have:

What's the difference between device storage and iCloud storage? - Apple Support

See how much iCloud storage you have

1. Go to Settings.
2. Tap [your name].
3. Tap iCloud. 
4. 
   
5. 

The amount of iCloud storage that you've used is listed at the top.

Good luck!