You are receiving this error because all of the seats on the license are currently being used.
You can login to your account on our website to manage your registrations: http://www.productivecomputing.com/licenseManager.php
Alternatively, you can contact us at support@productivecomputing.com and let us know the name of the machine that you would like to deactivate as well as the name of the machine that you would like to register and we can make the necessary changes. The machine name can be found by right clicking on “My Computer” selecting “Properties” and navigating to the “Computer Name” tab.