Yes, the product key has been registered already to your user account and cannot be registered twice. To activate it on another system..
1. Go to menu Studio One > Studio One Activation (if it launches directly into the activation screen go to step 2)
*** If you've never run Studio One on that system before it may launch directly into a log in screen or it may pop up later. . If so, be sure to log in with the credentials for the user account where the product is or activation will not work. Step 3 below will not list your product. ***
2. Check the option ... Activate Do not check the box "I have purchased an upgrade of Studio One”
3. Click the downward facing arrow on the right edge of the list field there to refresh your product keys listing with Show My Existing Product Keys. This will retrieve the registered products from your user account.
4. Select the product from the refreshed list and click Activate and it will activate the new license and restart as that product.