Registrations made through a cloud relay service (i.e. Apple iCloud Private Relay), VPN or Proxy are most likely to be auto-rejected, thus causing your preferred username and email address to be forfeited. You will have to register again without using the services mentioned above.
How To Disable Apple Private Cloud Relay service
If your registration was rejected or your account was disabled, and you did not use any of the services mentioned above, this could be because your account was flagged as a spammer OR is in violation of one or more of our Community Guidelines/Terms of Service.
Reference 1: Community Guidelines - One Account
Reference 2: Community Guidelines - Usernames, Avatars, Signatures, and Profiles
As part of our efforts to protect the community from spam and scammer activity, we have instituted these policies.