PusherBroadcaster error. Laravel 6 - How to Fix?

Question

I have Laravel 6 and use pusher as a broadcast event. But I get this error (see screenshot)

the error \Illuminate\Broadcasting\Broadcasters\PusherBroadcaster.php:121

screenshot stacktrace error

here is my controller:

public function sendMessage(Request $request)
{
     //validate incoming request
     $this->validate($request, [
         'message' => 'required|string'
     ]);

     try {
         $iam = Auth::user();
         $message = $iam->family->messages()->create([
            'message' => $request->input('message'),
            'id_user' => $iam->id
         ]);

         event(new ChatSubmitted('$message'));

         //return successful response
         return response()->json(['messages' => $request->all(), 'message' => 'Send Message Succesfully'], 200);
      } catch (\Exception $e) {dd($e);
          //return error message
          return response()->json(['message' => 'Send Message Failed!'], 409);
      }
  }

and here is my event:

class ChatSubmitted implements ShouldBroadcast
{
    use Dispatchable, InteractsWithSockets, SerializesModels;

    public $message;

    /**
     * Create a new event instance.
     *
     * @return void
     */
    public function __construct($message)
    {
        $this->message = $message;
    }

    /**
     * Get the channels the event should broadcast on.
     *
     * @return \Illuminate\Broadcasting\Channel|array
     */
    public function broadcastOn()
    {
        return new Channel('my-channel');
    }
}

the routes of api:

Route::get('messages', 'ChatsController@fetchMessages');
Route::post('messages/create', 'ChatsController@sendMessage');