FortiOS 6.0.8 and later versions only support the à la carte FortiClient Telemetry license available in batches of 100 clients. This license is compatible with FortiOS 5.6.0 and later versions. FortiOS 6.0.8 does not support other older FortiOS-side Telemetry and Compliance licenses.
If upgrading FortiOS 5.4 or 5.6 with a license other than the à la carte license to 6.0.8, FortiOS retains the license until its expiry, after which you must purchase the à la carte FortiClient Telemetry license. Contact your Fortinet sales representative for information.
For a video about applying FortiClient licenses to FortiGate, see the How to Purchase or Renew FortiClient Endpoint Subscription video.