Cannot assign Public GameObject variable in Inspector...

I am creating a script that makes an object rotate after certain time has pass.

Code looks like this:

 public GameObject Left01, Left02, Left03;
 public float countdown = 4.0f;

 void Update () 
 {
     countdown -= Time.deltaTime;
     if (countdown <= 3.0f) {
         Left01.transform.Rotate (new Vector3 (0, 10, 0) * Time.deltaTime);
     }

     if (countdown <= 2.0f) {
         Left02.transform.Rotate (new Vector3 (0, 10, 0) * Time.deltaTime);
     }

     if (countdown <= 1.0f) {
         Left03.transform.Rotate (new Vector3 (0, 10, 0) * Time.deltaTime);
     }
 }

}

The variable slots appear in the inspector no problem, but when I try to drag the object into the slot, nothing happens. The object is not recognized and I cannot drop in where it belongs.

Is the anything wrong with my object? Its being properly tagged.