Ahh, you can’t run a Panel just by itself. What you probably want to do is either create a JFrame and redesign the whole panel inside of it, or just add your panel to the palette, then create a JFrame, and put your panel in the middle of it, then run the Frame.
If you don’t want to do any of that, you are certainly welcome to write your own main method in your JPanel class that creates a frame and puts your panel inside of the content pane, then displays itself. That’s another option.
The point being that you need a Frame at some point, and by default, the JFrame template gets a Main method ot make it easier to run when using M4M, so that might be faster for you.