Stopping camera

You should use GameObject.Find("name"). But there are other errors: mainCamera and firstPersonControllerCamera types should be MouseLook, not GameObject. Additionally, you should assign them only once (at Start, for instance) in order to avoid possible hiccups when pressing F (Find functions are slow). Since the First Person Controller prefab has the tag "Player", is easier and faster to find it by tag. The main camera is easier: the Camera.main variable is a Camera reference that shows which's the main camera.

private MouseLook firstPersonControllerCamera; private MouseLook mainCamera;

     void Start(){
         // It's easier to find the FPC by its tag:
         GameObject player = GameObject.FindWithTag("Player");
         firstPersonControllerCamera = player.GetComponent< MouseLook>();
         // No need to Find the main camera: use Camera.main instead:
         mainCamera = Camera.main.GetComponent< MouseLook>();
     }

     void Update (){
         if(enter == true){
            if(Input.GetKeyDown("f")){
                TalkingMenu = true;
                Screen.lockCursor = false;
                firstPersonControllerCamera.enabled = false;
                mainCamera.enabled = false;
            }
         }
     }