Can't use prefab on my player.

Trying to play around with unity so I made a simple player controller, and a weapon controller. I drag my Weapon Controller onto my player, and I try to assign a Prefab as the starting Gun, but for some reason it doesn't let me do it. I can't drag the Prefab to the startingGun in my script.

My weapon controller looks like this:

 namespace Assets.Scripts.Weapon
 {
     public class WeaponController : MonoBehaviour
     {
         public Transform WeaponHold;
         public Gun startingGun;
         private Gun EquippedGun;
 
         void Start()
         {
             if (startingGun != null)
             {
                 EquipWeapon(startingGun);   
             }   
         }
 
         public void EquipWeapon(Gun gunToEquip)
         {
             if (gunToEquip != null)
                 Destroy(EquippedGun.gameObject);
 
             EquippedGun = Instantiate(gunToEquip, WeaponHold.position, WeaponHold.rotation) as Gun;
             if (EquippedGun != null)
                 EquippedGun.transform.parent = WeaponHold;
         }
     }
 }