"The type or namespace name 'UnityWebRequest' could not be found." Version 5.5.2p2

 

You need to include the UnityEngine.Networking namespace (not Unity.Networking)

 using UnityEngine.UI;
 using UnityEngine.Networking;

public void Subscribe() {
        Email = email.GetComponent<InputField>().text;
        var n = new JSONObject();
        n["entry_1064445353"] = Email;
        string json = n.ToString();
        Debug.Log(Email);
        StartCoroutine(Post(json));
    }