BCE0053: Property 'UnityEngine.Component.light' is read only.

Hello, I'll make it short. Why am I getting this error :

BCE0053: Property 'UnityEngine.Component.light' is read only.

This is my code :

 var sun : Light;
 var enableShadows : boolean = true;
 
 function Start()
 {
 enableShadows = true;
     if(enableShadows == true)
     {
         sun.light = LightShadows.Hard;
     }
 }

This is just a small part of my code, but the other part doesn't have anything to do with this one.

Thank you.