C# winform application sql connection open not working (A network-related or instance-specific error occurred )

[System.Data.SqlClient.SqlException] = {"A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (p...

private void AddClient()
        {
            

            string ConnectionString = "Data Source= ./SQLEXPRESS;Initial Catalog=iPhotographer;Integrated Security=SSPI";

            string sqlInsert = "INSERT INTO Clients(Client, Contact, Address, Telephone, NumberOfJobs, LastJob)VALUES(@Client, @Contact, @Address, @Telephone, @Jobcount, @Lastjob)";

            SqlConnection conn = new SqlConnection(ConnectionString);

            try
            {
                conn.Open();
                SqlCommand cmd = new SqlCommand(sqlInsert, conn);
                cmd.CommandType = CommandType.Text;
                cmd.Parameters.AddWithValue("@Client", this.Client);
                cmd.Parameters.AddWithValue("@Contact", this.Contact);
                cmd.Parameters.AddWithValue("@Address", this.Address);
                cmd.Parameters.AddWithValue("@Telephone", this.Telephone);
                cmd.Parameters.AddWithValue("@Jobcount", this.NumberOfJobs);
                cmd.Parameters.AddWithValue("@Lastjob", this.LastJob);
                cmd.ExecuteNonQuery();

            }
            catch (Exception error)
            {
                MessageBox.Show("Error: " + error);
            }

            finally
            {
                conn.Close();
            }
            MessageBox.Show("A new client has successfully been added to the client database.");

            pnlClients.Visible = false;
        }

        private void NewClientDetails()
        {
            this.Client = tbClient.Text;
            this.Contact = this.tbContact.Text;
            this.Address = this.tbAddress.Text;
            this.Telephone = int.Parse(this.tbTelephone.Text.ToString());
            this.NumberOfJobs = int.Parse(this.tbJobs.Text.ToString());
            this.LastJob = DateTime.Parse(this.dtpLastJob.Text.ToString());
        }