Your product key is associated with the email address to access Sophos Home. You can activate the product key only at one time. After activation, it will select Sophos home account. However, sometimes the product key may not work and show an error message saying, "The key is already used." The error results when you try to activate a product key that has already been activated, as the product key can only be activated once.
There are two main methods to fix the product key not working issue.
In this method, log into your previous premium-activated account and re-protect your devices.