Apple ID
I'm trying to change my Apple ID but am getting the message "This email address is not available." Can you help me?
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It means the email address you are trying to use is already in use or was used on another Apple ID. So it cannot be used.
Link-> Change your Apple ID - Apple Support
If you see a message that the email address you want to use isn't available or is already in use, check that you or a family member aren't already using that email address with a different Apple ID. If a family member is using that email address, remove it from their account then try to use it.
Try a different email address.