If you received a message that the number you are trying to attach to your account is already associated with another Twitter account, it’s possible that your phone number previously belonged to someone else.
Don’t worry! This only means that outdated information still exists in the system. This does not mean that the previous owner has any access to the Twitter account you create with your new phone number. If you are not able to create an account with your new phone number, try creating an account using your email address.