It says my phone number is already associated with another account?

See this Apple article: Deregister iMessage on your iPhone or online

Deregister iMessage on your iPhone or online – Apple Support (UK)

I try it myself but they didn't verification code

Sign in to your Apple ID account page and edit / update your Apple ID details.

https://appleid.apple.com

-> edit your trusted phone number.

But I’ll try it with my other iCloud

So you use / have multiple Apple ID.

And that number is still linked to that other iCloud account.

Do you use multiple Apple ID?

Do you have another iPhone?

I just got the number,

You get that 'new' number from your carrier?

Check with your carrier whether that telephone number has been assigned to you.

It is possible that your 'new' number is 'known' to Apple, may have been used/linked to another Apple ID.

Contact your carrier.

Ask: what or which cooling-off period they use.

An explanation of cooling-off period in my region:

There is a difference: by law there is no 'cooling off period'. 

Vodafone writes in a response that original Vodafone numbers, ie numbers purchased by Vodafone from ACM (Authority for Consumers & Markets), will go back into circulation after approximately nine months; six months applies to T-Mobile. Hi and Tele2 are talking about three months. KPN indicates that there is no term, which means that 'a telephone number that becomes available today may possibly be issued again tomorrow'. The procedure after terminating a subscription is always the same, whether it concerns death or other reasons.

I’ve not use for iMessage before so I don’t why they are telling me that it has been use by another device

Can you help me to disregister from the other phone,

Why the contradictory information you provide to us?

I did ask you:

Do you use multiple Apple ID?

Do you have another iPhone?

And you didn't give an answer.

Do you suddenly get that message?

Do you use multiple Apple ID?

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