You need a separate license key for the failover setup, and it needs to be activated on the command line of the secondary server to ensure you don't activate the separate license key on the wrong server. The message you are getting indicates that you are trying to use a license key that is activated already on the primary machine, on another machine which does not have the same hardware specifications and therefore fails. Please contact the support ticket system at
www.openvpn.net for more information.