I created WCF services when I run it..its showing below error

[OperationContract]
[WebGet(UriTemplate = "GetUserLogin/{UserID}/{Password}", RequestFormat = WebMessageFormat.Json, ResponseFormat = WebMessageFormat.Xml)]
Users GetUserLogin(string UserID, string Password);
Note: the above code I wrote in interface

public Users GetUserLogin(string UserID, string Password)
{
Users us = new Users();
SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["constr"].ToString());
con.Open();
SqlCommand cmd = new SqlCommand("spUserLogin", con);
cmd.CommandType = CommandType.StoredProcedure;
cmd.Parameters.AddWithValue("@UserID", UserID);
cmd.Parameters.AddWithValue("@Password", Password);
SqlDataAdapter da = new SqlDataAdapter(cmd);
con.Close();
DataSet ds = new DataSet();
da.Fill(ds, "tb_Users");

return us;
}
note; this above code I wrote in svc class after that when I run this code I got above error