Why is my iCloud does not backuping

Why is my iCloud does not back up?

Looks like because your iPhone is oudated.

See this Apple article: System requirements for iCloud

System requirements for iCloud – Apple Support (UK)

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iCloud+

iCloud+ is available on iPhone, iPad or iPod touch, Mac, PC and web browsers.

• iOS 15 or iPadOS 15

When you turn iCloud On, you get 5 GB iCloud storage free.

iPhone 6 Plus capacity -> 16 GB or 64 GB or 128 GB.

You need te back up to your computer, see this Apple article: How to back up your iPhone, iPad, and iPod touch

How to back up your iPhone, iPad, and iPod touch - Apple Support