iCloud show 50gb free, backup size 18gb but still failed to backup

Hi Bonnielian530,

Based on your screenshot it looks like the backup currently saved is 18.3 GB. If you tap on that backup, you should see what is projected as your next backup size. Does this exceed the amount of available storage? If so, you'll need to manage your iCloud storage and free up space before backing up your data.

Check out more information here: If you can't back up to iCloud - Apple Support

Take Care!