Fatal error: Uncaught Error: Class "Response" not found

That's PHP basics.

When you want to use a class, you need to declare it.

use Response;
...
if ($note['user_id'] != $currentUserId) {
   return abort(Response::FORBIDDEN);  
}

To avoid any conflict, I would probably use another class name because response is already used in the framework.

@shivamyadav Try this code

  namespace your_file_path;

  class Response{
    const NOT_FOUND = 404;
    const FORBIDDEN = 403;
 }

namespace like be

     namespace App\services\Response;

And where you want to use this class use namespace path like

    use App\services\Response;