Complete these steps
1) In config/app.php uncomment this line -
App\Providers\BroadcastServiceProvider::class,
2) If the above does not work , and you still have an error - then I'm sure that you are not logged in . Just log in to your account and check (because this is your private broadcast).
This will solve your problem, and since you can also pass additional auth parameters as you need in app\Providers\BroadcastServiceProvider.php
Broadcast::routes(['middleware' => 'auth:admin']);
source share