Out of the box, your FortiGate device will most likely be in Switch mode, this groups the internal interfaces in to a single switch. To change to Interface mode, follow the below steps.
Interface mode allows you to control all of the interfaces separately. To change modes make sure none of the interfaces (lan/internal) are referenced
Enter Interface Mode
config system global set internal-switch-mode interface end
Enter Switch Mode:
config system global set internal-switch-mode switch end