C# not recognizing packages

mcs Program.cs
Program.cs(6,7): error CS0246: The type or namespace name `RestSharp' could not be found. Are you missing an assembly reference?
Compilation failed: 1 error(s), 0 warnings

using System;
using System.Collections.Generic;
using System.Text;
using System.Linq;
using System.Threading.Tasks;
using RestSharp;
namespace Main
{
    class Program 
    {
        static void Main(string[] args)  {
            

                
            }

        }

    }