You cannot use FortiManager to configure high availability (HA) on FortiGate devices. FortiManager learns about HA settings from managed FortiGate devices, but does not manage that part of the FortiGate configuration. As a result the HA pane in the device database is read-only in Device Manager.
It is recommended to configure HA settings directly on FortiGate units. Another option is to run CLI scripts directly against FortiGate devices.