Show values of a List in Labels

It does not show you anything because you're simply storing the data in a variable list2 . To show the data you must include the data in a control within a form.

Look at this example:

    private void getValues2()
    {
        List<string> list2 = new List<string>();
        cn.Open();
        SqlCommand cmd = new SqlCommand("SELECT  SUM(Qty) AS QTY FROM    tbl_Kanban GROUP BY Part_Num", cn);
        SqlDataReader leer;
        leer = cmd.ExecuteReader();
        while (leer.Read() == true)
        {
            list2.Add(leer[0].ToString());
        }
        if (list2.Count > 0)
        {
            ShowData(list2);
        }
        cn.Close();
    }

    private void ShowData(List<string> data)
    {
        var form = new Form();
        var listbox = new ListBox();
        form.Controls.Add(listbox);
        listbox.Items.AddRange(data.ToArray());
        form.Show();
    }

The ShowData method receives a list of strings, creates a form, adds a ListBox control to the form, and includes the list data in the ListBox. Finally show the form.

The correct thing would be for the function to return the list as an answer:

private List<string> getValues2()
{
    cn.Open();
    SqlCommand cmd = new SqlCommand("SELECT  SUM(Qty) AS QTY FROM tbl_Kanban GROUP BY Part_Num", cn);
    SqlDataReader leer = cmd.ExecuteReader();

    List<string> lista = new List<string>();
    while(leer.Read())
    {
        lista.Add(leer[0].ToString());
    }

    cn.Close();

    return lista;
}

Then later in the form you would use:

List<string> lista = getValues2();

foreach(var item in lista)
{
    ListBox1.Items.Add(item);
}

I do not know if getValues2() define it within a class or in the same form , but the idea is to separate responsibilities, on the one hand you get the data and on the other you assign these to ListBox .

If that function is in a class you should create the instance to invoke its functionality.