If the email address is linked to your organisation (e.g. email@example.com), the user may have signed up via the FlourishDx webpage instead of using your organisation's registration link, or they may have intentionally registered a basic account at some time in the past. If it is a personal email address, they may have been registered with a previous employer.
In any of these situations, the email address is registered in an account for which you do not have Owner or Admin permissions. This means that you cannot resolve the issue within your account. Please lodge a ticket at support.flourishdx.com