Why I am getting (are you missing assembly references) error

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;
using MVC1.Models;
namespace MVC1.Controllers
{
    public class customer
    {
        public string customername { get; set; }
        public string address { get; set; }
        public string phone { get; set; }
        public override string ToString()
        {
            return this.customername + "" + this.address + this.phone;
        }  
    }
    public  class employee
    {
        public string Firstname { get; set; }
        public string LastName { get; set; }
        public int budget { get; set; }
    }
    
    public class TestController : Controller
    {
        public ActionResult GetView()
        {
            employee emp = new employee();
            emp.Firstname = "KARAN";
            emp.LastName = "Kumar";
            emp.budget = 30000;
            ViewData["employee"] = emp;
            return View("View");
        }
        public customer Getcustomer()
        {
            customer cus = new customer();
            cus.customername = "AMAN";
            cus.address = "   From Srilanka ";
            cus.phone = " 456478546";
            return cus;
        }
	}
}

using MVC1.Models;