Each FortiGate unit in a cluster must have its own FortiClient license. Contact your reseller to purchase FortiClient licenses for all of the FortiGate units in your cluster.

When you receive the license keys you can log into https://support.fortinet.com and add the FortiClient license keys to each FortiGate unit. Then, as long as the cluster can connect to the Internet each cluster unit receives its FortiClient license key from the FortiGuard network.