Build failed. Method not found?

Build failed. Method not found: 'Int32 System.Environment.get_CurrentManagedThreadId()'. Build: 1 error, 0 warnings'

What does this mean?

It is the error I get with this script:

using UnityEngine; using System.Collections;

public class Test : MonoBehaviour {

 // Use this for initialization
     void Start () {
     float speed = 10;
 }
 
 // Update is called once per frame
 void Update () {
     if (KeyCode.UpArrow){
         //transform.position += Vector3.up * Time.deltaTime; }
         transform.Translate(Vector3.right * speed * Time.deltaTime);
     }
 }