SetActive canvas relative to selected object

I'm trying to SetActive the canvas relative to the position of selected gameobject. but canvas will always appear relative to the position of first object I attached the script to. here's an example script:

 public GameObject menu;
 public GameObject obj;
 void Start()
 {
   menu.SetActive(false);
 }
 void OnSelect()
 {
    menu.SetActive(!menu.activeSelf);
    menu.transform.position = obj.transform.position + obj.transform.up *0.5f;
 }

not sure what went wrong. I drag all correct object accordingly but does not work properly. please help. thx a lot