There’s not enough iCloud storage available

"There’s not enough iCloud storage available: What should i do if cannot be backed up because there’s is not enough iCloud storage available"

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Troubleshooting iCloud Back Up:

A. Use a Computer, Instead:

Looking to Backup your iPhone? iCloud does not back up everything. So, it is best to back it up to a computer. See here: What does iCloud Back Up? - Apple Support

B. Download All of iCloud:

If you wanted to, you could download all of iCloud, and then trash what is in there. Once trashed, perform the back up.. Perform my User Tip: Getting a Quick and Direct "iCloud Drive-to-External Hard Drive" Download - User Tip. All would be saved, and would then be removed —photos and all. You would then use Finder to browse all of iCloud.

C. Clear Space:

Internal storage cannot be upgraded. It is what it is. So, the maximum internal storage capacity never increases, even if you upgraded your iCloud plan. Learn More here: What's the Difference Between Device Storage and iCloud Storage? Apple Support

So, what to do...

Clear Space Using my User Tips:

• How to Clear Space from your Mobile Device, and What to Clear: - User Tip. You may have too much installed, and need to clear space.
• How to Clear "Other" in your iPhone's Storage - Use Backup - User Tip. Caches would be being used for remembrance of application use. It is essentially the same for an iPhone. You'd backup your iPhone, and then restore your iPhone from the backup you've just created. All System Data will delete.