There are some occasions when a machine with an activated license cannot be accessed to deactivate the license. Then, you can manually delete the old key and re-generate a new one.
My machine was freshly imaged or replaced and I need to re-activate my license on the new machine. When I try to re-activate my new machine I have 0 seats available.
Log on to your Telerik.com account.
Navigate to Test Studio product.
Find and delete your old key. This will free up a slot allowing to activate your new machine.