In EdReady, you can either have a student account, or an administrative (teacher) account, but you cannot have both types of accounts for the same identity.
In the course of creating a new administrative user, you may encounter an error that states: "A (Student) user with this email address already exists and is associated with the following sites: [site name 1], [site name 2], etc." This error tells you the person you are trying to add already has an EdReady student account. Usually the person will have created the account in order to test out and explore EdReady from the student perspective. As long as that student account exists, the person will not be able to also have an administrative account using that same email address.
One solution is for the person to use a different email address. By using two distinct email addresses, a person can retain both a student account and an administrative account. They will have to remember and manage both addresses accordingly.
Another solution is to delete the student account. Contact your NROC support staff (e.g., implementation specialist) and that person can delete the account for you. Once the account has been deleted, the email address will become available for re-designation as an administrative account.