C# error 127.0.0.1:5432 that cannot connect to PostgreSQL Server

I need help on connecting to my friend's PostgreSQL server through my team's project using C# and WinForms in VS Community with Npgsql as the NuGet Package. The problem always shows when we try to connect to his server and it will not allow us to login or register new users to the database, but on the other note, my friend has his server installed in his laptop, and when he executes the program he can login using the data from database and register new users on it.

Here is our code

public static class ConnectionManager
{
    private static NpgsqlConnection connection;

    public static NpgsqlConnection Connection
    {
        get { return connection; }
    }

    public static NpgsqlConnection GetNpgsqlConnection()
    {
        string connectionString = "Host=localhost;Username=postgres;Password=Kirkdave123;Database=MSAD_Project";

        connection = new NpgsqlConnection(connectionString);
        connection.Open();

        return connection;
    }

    public static void TurnoffNpgConnection()
    {
        connection.Close(); 
    }
}

(Login Form Code)
public partial class LoginForm : Form
{
    private bool isFirstClick = true;
    public LoginForm()
    {
        InitializeComponent();
    }

    private void LoginBtn_Click(object sender, EventArgs e)
    {
        // Will check in database if username and password is a user, or an admin.

        string username = UserName_txtbx.Text;
        string password = Password_txtbx.Text;

        if (string.IsNullOrWhiteSpace(username) || string.IsNullOrWhiteSpace(password))
        {
            MessageBox.Show("Please enter both username and password.", "Login Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
            return;
        }

        try
        {
            using (NpgsqlConnection connection = ConnectionManager.GetNpgsqlConnection())
            {
                string query = "SELECT COUNT(*) FROM public.\"User_Account\" WHERE \"Username\" = @username AND \"Password\" = @password";
                using (NpgsqlCommand command = new NpgsqlCommand(query, connection))
                {
                    command.Parameters.AddWithValue("@username", username);
                    command.Parameters.AddWithValue("@password", password);

                    int count = Convert.ToInt32(command.ExecuteScalar());
                    if (count > 0)
                    {
                        MessageBox.Show("Login successful!", "Login", MessageBoxButtons.OK, MessageBoxIcon.Information);
                        var dashboard = new DashboardForm();
                        dashboard.Show();
                        this.Hide();
                    }
                    else
                    {
                        MessageBox.Show("Invalid username or password.", "Login Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
                    }
                }
            }
        }
        catch (Exception ex)
        {
            MessageBox.Show("An error occurred during login: " + ex.Message, "Login Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
        }
    }

    private void ForgotPasswordLinkLbl_LinkClicked(object sender, LinkLabelLinkClickedEventArgs e)
    {
        var forgotpassword = new ForgotPasswordForm();
        forgotpassword.Show();
        this.Hide();
    }

    private void RegisterLinkLbl_LinkClicked(object sender, LinkLabelLinkClickedEventArgs e)
    {
        var register = new RegisterForm();
        register.Show();
        this.Hide();
    }

    private void UserName_txtbx_Click(object sender, EventArgs e)
    {
        UserName_txtbx.Clear();
    }

    private void Password_txtbx_Click(object sender, EventArgs e)
    {
        Password_txtbx.Clear();
    }
}

(Register Form Code)
public partial class RegisterForm : Form
{
    private Random random = new Random();

    public RegisterForm()
    {
        InitializeComponent();
    }

    private void GobackBtn_Click(object sender, EventArgs e)
    {
        var login = new LoginForm();
        login.Show();
        this.Hide();
    }


    private void RegisterBtn_Click(object sender, EventArgs e)
    {
        bool CheckBox1Checked = CheckBox1.Checked;

        if (!CheckBox1Checked)
        {
            MessageBox.Show("You need to know the Policy and Term's and Condition's" + "before registering");
        }

        else if (!ValidateFormFields())
        {
            MessageBox.Show("Please fill in all the required fields.", "Registration Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
        }

        else
        {
            string username = UsernameTbx.Text;
            string password = PasswordTbx.Text;
            string email = EmailTbx.Text;
            string phone = PhoneTbx.Text;
            string lastName = LNametxtbx.Text;
            string firstName = Fnametxtbx.Text;
            int age = Convert.ToInt32(AgecomboBox.SelectedItem);
            string gender = GcomboBox.SelectedItem.ToString();

            int userID = GenerateRandomUserID();

            try
            {
                using (NpgsqlConnection connection = ConnectionManager.GetNpgsqlConnection())
                {
                    string query = "INSERT INTO public.\"User_Account\" (\"UserID\", \"Lastname\", \"Firstname\", \"Gender\", \"Age\", \"Email\", \"Phonenumber\", \"Username\", \"Password\") " +
                        "VALUES (@userID, @lastname, @firstname, @gender, @age, @email, @phone, @username, @password)";

                    using (NpgsqlCommand command = new NpgsqlCommand(query, connection))
                    {
                        command.Parameters.AddWithValue("@userID", userID);
                        command.Parameters.AddWithValue("@lastname", lastName);
                        command.Parameters.AddWithValue("@firstname", firstName);
                        command.Parameters.AddWithValue("@gender", gender);
                        command.Parameters.AddWithValue("@age", age);
                        command.Parameters.AddWithValue("@email", email);
                        command.Parameters.AddWithValue("@phone", phone);
                        command.Parameters.AddWithValue("@username", username);
                        command.Parameters.AddWithValue("@password", password);

                        int rowsAffected = command.ExecuteNonQuery();

                        if (rowsAffected > 0)
                        {
                            MessageBox.Show("Registration successful!", "Will Direct You to Login Form", MessageBoxButtons.OK, MessageBoxIcon.Information);
                            var login = new LoginForm();
                            login.Show();
                            this.Hide();
                        }
                        else
                        {
                            MessageBox.Show("Registration failed!", "Registration Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
                        }
                    }
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show("An error occurred during registration: " + ex.Message, "Registration Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
            finally
            {
                ConnectionManager.TurnoffNpgConnection();
            }
        }

    }

    private int GenerateRandomUserID()
    {
        return random.Next(1000000, 9999999);
    }

    private bool ValidateFormFields()
    {
        if (string.IsNullOrWhiteSpace(UsernameTbx.Text) ||
            string.IsNullOrWhiteSpace(PasswordTbx.Text) ||
            string.IsNullOrWhiteSpace(EmailTbx.Text) ||
            string.IsNullOrWhiteSpace(PhoneTbx.Text) ||
            string.IsNullOrWhiteSpace(LNametxtbx.Text) ||
            string.IsNullOrWhiteSpace(Fnametxtbx.Text) ||
            AgecomboBox.SelectedItem == null ||
            GcomboBox.SelectedItem == null)
        {
            return false;
        }
        return true;
    }

    private void pictureBox6_Click(object sender, EventArgs e)
    {

    }
}

and when they run it shows this error

We already tried on changing the pg_hba.conf and postgresql.conf and also set up the Windows Defender Firewall for inbound rule on his laptop, but we still cannot connect to the PostgreSQL server, is this because of the C# code or is it more on network connectivity?