TypeLoadException with libraries in C#

Hello, I've searched the internet for the past 2 days to find out myself, but I didn't find anything useful or up to date for my problem, so here it is:

I am currently working on a 2D MMORPG nothing really too important, but the thing is, I want to connect to the database, (what actually should work) But I happen to have this strange error, (I can actually start the game tho).

Here's the beginning of the DB.cs (where the database stuff is)

Error:

TypeLoadException: Could not load type 'System.Data.SqlClient.SqlConnection' from assembly 'System.Data, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089'.

 using UnityEngine;
 using System.Collections;
 using System.Data;
 using MySql.Data.MySqlClient;

If you need any other codelines tell me.