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See this read: iCloud: The maximum number of free accounts have been activated

https://appletoolbox.com/icloud-the-maximum-number-of-free-accounts-have-been-activated-qa/ The three accounts rule is a function of Apple hardware, not software.

Create a new ID (if necessary) and sign into iCloud using the same Apple ID on another Apple device/computer. You should then be able to sign in.