need help wit unity and mysql

after i declare some variables of connection to msql i get an error Internal compiler error : See the console log for more information. output was: Unhandled Exception: Mono.CSharp.InternalErrorException: Assets/test.cs(10,14): test ---> Mono.CSharp.InternalErrorException: Assets/test.cs(11,33): test.testa ---> System.IO.FileNotFoundException: Could not load file or assembly 'System.Drawing, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies.

     using UnityEngine;
     using System.Collections;
     using System;
     using Mono.Data.Sqlite;
     using System.Data;
     using System.Data.SqlClient;
     using System.Collections.Generic;
     using MySql.Data.MySqlClient;
     using System.IO;
     public class test : MonoBehaviour {
         private MySqlConnection testa ;
         private string server;
         private string database;
         private string uid;
         private string password;
         // Use this for initialization
         void Start () {
     
         }
         void Update () {
             
         }
         }
 

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