string QueryCon = "SELECT Id_Categoria FROM Categoria where Nombre='" + Categoria + "'";
            connection.Close();
            connection.Open();
            MySqlCommand ConnCombo = new MySqlCommand(QueryCon, connection);
            MySqlDataReader reader = ConnCombo.ExecuteReader();
            while (reader.Read())
            {
                Id_C = int.Parse(reader.GetString("Id_Categoria"));
            }

            string query = "INSERT INTO compra (Articulo, Categoria,Importancia,Obligatorio,Costo,Descripcion,Adquirir) VALUES('" + Articulo + "', '" + Id_C + "'," + Importancia + ",'no'," + Costo + ",'" + Descripcion + "','No','" + Mes + "')";

            if (aprobar==true) {
                //create command and assign the query and connection from the constructor
                MySqlCommand cmd = new MySqlCommand(query, connection);

                //Execute command
                cmd.ExecuteNonQuery(); ------- Error

                connection.Close();
            }