Downgrade iCloud

Did you follow the steps mentioned in this article to do so?

Downgrade or cancel your iCloud+ plan - Apple Support

Also check this requirement:

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Before you downgrade or cancel your iCloud+ plan, first download or remove content that exceeds your new storage amount. To make sure that you don't lose any information, learn how to copy what you store in iCloud.

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copied from: Downgrade or cancel your iCloud+ plan - Apple Support

If you previously clicked it successfully, wait until the current billing cycle ends. Otherwise, see the section about that issue in the article Ingo2711 linked to.

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