This warning is due to license already being activated:
To resolve this, follow the steps below:
1. Go to your account management located at http://www.pragmaticworks.com
2. Select the "My Account"
3. Log In and select the Registered Products Tab
4. Expand the arrow next to the product you are trying to register and it will list all the following information, here you will identify the environment identifier and key you would like to deactivate.
5. After looking at this information you find that you can deactivate a license, you can go through the help page of the Workbench. If that user is no longer with the organization, you can contact Pragmatic Works Support via email with the Environment Identifier and Environment Key.