How to set Panel opacity ...

You can't set an opacity for a panel: it's a function of top level windows (such as forms) only - it is a hardware function that isn't enabled for child windows such as Panel.

You can - sort of - do it, by setting the panel background colour to an Alpha value of zero, but...it's not very good!

public frmMain()
    {
    InitializeComponent();
    myPanel.BackColor = Color.FromArgb(0, Color.Black);
    }
protected override void OnPaint(PaintEventArgs e)
    {
    e.Graphics.DrawLine(Pens.Red, 0, 0, Width, Height);
    }