Huge purple rectangle on my screen

Hello,

So being new to the community and the world of programming, I just wanted to ask why my code didn't work? I added a .cs class file into my plugin folder and reloaded the server with this code:
namespace Oxide.Plugins

Code:

{
    [Info("Title of Plugin", "Description", "Your Name", "0.1.0", ResourceId = 681)]
    public class PluginName : RustPlugin
    {
        void OnPlayerSpawn(BasePlayer player)
        {
            var message = "You've respawned from a terrible death";
            rust.SendChatMessage(player, message);
        }
    }
}

Instead of popping up a message it displays a huge purple rectangle at the top right of my screen, as displayed below: