When attempting to activate a licence key on a new domain, the following message is displayed:
"It is not possible to register another website with this license key. Remove the license key from another website to register it here. Check your license key in our customer portal service.borlabs.io."
This message appears when the quota of a licence has been exhausted. To activate the license on a new website, you must first remove an existing website from the license in the customer portal at https://service.borlabs.io/login.
Then click on "Licences" or "Borlabs Cookie" and there on the gear icon for your licence:

In the licence details, open the "Websites" tab at the top.

Click the icon in the "Remove" column. You can then activate the licence key on the new website.