Don´t panic, usually this is a user you registered years ago and have not used since 🙌
If you can still access the registered number or email to the account you can just follow this guide to reset the password and delete your old account 🗑
Once your old account is deleted, your phone number will be released and you can easily register it to your current account 🤝
If your account is registered with an old phone number or email, contact support for further help to register your new contact information 🗣