error CS0309: The type `string' must be convertible to `UnityEngine.Object' in order to use it as parameter `T' in the generic type

I having a problem cloning my prefab. I don’t want my prefab clone in different locations in the scene. When I save my script I got this error CS0309: The type `string’ must be convertible to

UnityEngine.Object' in order to use it as parameter T’ in the generic type or method

`UnityEngine.Object.Instantiate(T)’

Here is my script :

using UnityEngine;
using System.Collections;

public class MudFam : MonoBehaviour {

public GameObject gameObject = null;
	// Use this for initialization
	void Start () {
	GameObject gameObject = Instantiate("Prefab/prefab_Mud") as GameObject;
	}
	void Update () {
	
	}
}

You can’t do that, if you want to instantiante a prefab then assign your prefab to a variable in your inspector

     public GameObject prefab; //Assign your prefab in inspector
     // Use this for initialization
     void Start () {
     GameObject gameObject = Instantiate(prefab) as GameObject;
     }
     void Update () {
     
     }

You must first load it with Resources.Load(“Prefab/prefab_Mud”) and then Instantiate it.
And Resource.Load() only searches for assets in the Resources folder, is your prefab in Resources folder?