MySql C# compiler error

So I have the following code

 using UnityEngine;
 using System.Collections;
 using MySql.Data.MySqlClient;
 
 public class NewBehaviourScript : MonoBehaviour {
 
     // Use this for initialization
     void Start () {
         
         string server = "localhost";
         string database = "mysqldatabase1";
         string uid = "username";
         string password = "password";
         string connectionString;
         connectionString = "SERVER=" + server + ";" + "DATABASE=" + 
         database + ";" + "UID=" + uid + ";" + "PASSWORD=" + password + ";";
 
         MySqlConnection connection = new MySqlConnection(connectionString);
     
     
     }
 
 }

When I compile the project it does not give any errors of warnings. However when I try to run the game I get the: "All compiler errors have to be fixed before you can enter play mode."

I have tested a bit and found out that the problem occurs when I add the using MySql.Data.MySqlClient; line.

Any ideas why this happens? Thanks in advance.