Cannot implicitly convert type 'string[]' to 'string' at return txtItems.ToArray(); line

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Services;
using MySql.Data.MySqlClient;
using System.Data;
using System.Web.UI.WebControls;
using System.Web.UI;

namespace autocomplete
{
///

/// Summary description for WebService1
///

[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[System.ComponentModel.ToolboxItem(false)]
// To allow this Web Service to be called from script, using ASP.NET AJAX, uncomment the following line.
// [System.Web.Script.Services.ScriptService]
public class WebService1 : System.Web.Services.WebService
{

[WebMethod]
public string GetCompletionList(string prefixText, int count)
{
MySqlConnection con = new MySqlConnection("Server=localhost;Database=projecttt;Uid=root;Pwd=xxxxx;");
DataSet ds = new DataSet();
DataTable dt = new DataTable();

MySqlCommand cmd = new MySqlCommand();
cmd.CommandType = CommandType.Text;

cmd.CommandText = "select username from processeddata_table Where username like @myParameter";
cmd.Parameters.AddWithValue("@myParameter", "%" + prefixText + "%");

try
{
con.Open();
cmd.ExecuteNonQuery();
MySqlDataAdapter da = new MySqlDataAdapter(cmd);
da.Fill(ds);
}
catch
{
}
finally
{
con.Close();
}
dt = ds.Tables[0];

//Then return List of string(txtItems) as result
List<string> txtItems = new List<string>();
String dbValues;

foreach (DataRow row in dt.Rows)
{
//String From DataBase(dbValues)
dbValues = row["username"].ToString();
dbValues = dbValues.ToLower();
txtItems.Add(dbValues);
}

return txtItems.ToArray();
}
}
}