0 references error throwing in VSTS selenium C# programming in Nunit

Question

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using NUnit.Framework;
using OpenQA.Selenium;
using OpenQA.Selenium.Firefox;

namespace FR_Sel_SupraNET
{
    [TestFixture]
    public class TRTests
    {
        //error:     When run this code its displaying 0 references before  [test]
        [Test]
        public void TCLogin()
        {
            string url = "https://kapsupranet01.test.suprakim.com/pages/kwm_203.aspx?Member=33";
            string loginname = "vs";
            string password = "supraqa1!";

            IWebDriver webdvr = new FirefoxDriver();
            webdvr.Navigate().GoToUrl(url);
            webdvr.FindElement(By.Id("txtUserName")).SendKeys(loginname);
            webdvr.FindElement(By.Id("txtPassword")).SendKeys(password);
            webdvr.FindElement(By.Id("btnLogIn")).Click();
        }

        [Test]
        public void TCmember()
        {
        }
    }
}

This is picture

This is quote

//error: When run this code its displaying 0 references before [test]

And fail to run this code and error is "The geckodriver.exe file does not exist in the current directory or in a directory on the PATH environment variable."