hidden icloud storage, how can i delete them?

If your using icloud backup, that backs up everything which is more than the basic 5 gb you have. You can backup contacts alone, but if your specifically talking about icloud backup, that backs up everything not just contacts. depends on how much things you have on the phone, but a backup might be anyware from 25gb or more. Point is, 5gb won't be enough to backup your phone to icloud. If you just want to backup contacts, turn on contacts, but that's separate from icloud backup.

Hi,
I can back up my iPhone with 5GB. Double tap on everything under the 'manage storage' option under iCloud and see what's there. When I've previously got a new phone and chosen to back up from iCloud, all my apps are there, and messages too, all with 5GB, and I'm not just talking about native apps, all my other apps too. Before you back up, delete your existing backup, which won't delete everything on your phone, and then try backing up.

Are you using the latest version of iOS? I don't understand why it's failing to backup when it appears you have enough space, and it doesn't seem like the space required for the backup is anywhere close to filling your quota. Perhaps try connecting to power and Wi-Fi, though this shouldn't be necessary if you want to manually trigger a quick backup.

This may be a longshot, but check the files app on the iPhone in question. If there are any files on there, documents, audio/video files, pictures, etc., those also will be stored in iCloud. While separate from iCloud backup, it still takes up storage space. Same with photos, if you are using a shared photo album, and especially if you have a ton of photos on iCloud Drive, that two takes up storage space.
Just a thought. ☺️

In addition to checking the files app under iCloud Drive, go to iCloud settings and check under the manage storage section. Some apps store data in such a way that it still uses your quota, but doesn't save the files in iCloud Drive itself. If you find things you don't want, delete the app data, but be very careful, as you may accidentally delete preferences and other important information used by apps. However, I don't think this is the problem. Using 56 MB is a drop in the bucket if you have 5 GB. TO be fair, 5 GB isn't a lot of space, but if you don't have a lot of content on your phone, you can easily make backups and stay well below this limit.