If you are unable to activate FusionReactor and on the about page you see the seats: no seats available, this indicates that you are currently using all available seats on your license key.

In order to see which servers are currently using your license, you can log in to the FusionReactor portal and view the activations for a license.

You can either purchase additional seats for your license to activate new servers or deactivate servers you are currently using FusionReactor on.