Try one of the following to resolve the issue:
- This error message means that your email address is associated with another Apple ID. It could be tied to GameCenter, iTunes Store, or another service. Try using your Apple ID for those services to log in to FaceTime or iMessage on your device.
- If you wish to use a separate Apple ID for FaceTime or iMessage, unlink the email address from any other Apple ID before using it as a separate Apple ID for FaceTime or iMessage.
- If you would like to change your Apple ID to another email address, go to My Apple ID and follow the instructions to change your Apple ID.