Troubleshooting Tip: Impossible to add interface to FortiLink

To add an interface to FortiGate, the interface should not have an IP configured and no reference on it. If that is configured and the interface still does not show in FortiLink, check the Interface settings.

If the Interface is set to DHCP, that can cause the issue as well:

Change it to manual and then the interface should show under FortiLink:

Note:
Another reason is that the interface is set as heartbeat interface under HA config. This will also show the reference as 0 on the interface itself.