i had error while debbuging a code in mono developunity editor i cant ressolve this error

C:\Windows\Microsoft.NET\Framework\v4.0.30319\Microsoft.Common.targets(5,5): Error MSB3645: .NET Framework v3.5 Service Pack 1 was not found. In order to target ".NETFramework,Version=v3.5", .NET Framework v3.5 Service Pack 1 or later must be installed. (MSB3645) (Assembly-CSharp)

using UnityEngine; using System.Collections;

public class PlayerController : MonoBehaviour { private Rigidbody rb; void Start() { rb = GetComponent(); }

 void FixedUpdate ()
 {
     float moveHorizontal = Input.GetAxis ("Horizontal");
     float moveVertical = Input.GetAxis ("Vertical");
     vector3 movement = new Vector3 (moveHorizontal,0.0f,moveVertical);
     rb.AddForce (movement);
 }

} this was mycode but i dont know where to solve .