Which authentication scheme is not supported by the RADIUS implementation on FortiGate?

CHAP (Challenge-Handshake Authentication Protocol) is a standard and supported authentication scheme used by RADIUS and implemented by FortiGate.

MSCHAP2 (Microsoft Challenge-Handshake Authentication Protocol version 2) is a common and more secure variant of CHAP, fully supported by RADIUS and FortiGate.

PAP (Password Authentication Protocol) is a basic authentication scheme that is supported by RADIUS and FortiGate, although it is considered less secure as credentials are sent in plain text.

FSSO (Fortinet Single Sign-On) is an entirely separate authentication solution that integrates with Active Directory for transparent user authentication, and it is not a RADIUS authentication scheme or protocol.