FortiEndpoint agents supported OS are listed in the below URL https://docs.fortinet.com/document/fortiendpoint/latest/administration-guide/393635/product-integration-and-support

Do you know if this device was a marked as "Excluded form Management " in the EMS. ? Try this : Go to EMS --> Endpoints --> All endpoints --> search for that device --> select --> Action --> Enable Management Or Have you enforced invitation only regi...