When upgrading to Drupal 9 I encountered this error a lot. In drupal 8 we used ResponseEvent for Event Subscribers, but in Drupal 9 this changed to RequestEvent.

Change

use Symfony\Component\HttpKernel\Event\GetResponseEvent;

 * @param GetResponseEvent $event
   * @return void
   */
  public function redirectMyContentTypeNode(GetResponseEvent $event) {

to

use Symfony\Component\HttpKernel\Event\RequestEvent;

// And further
**
   * Redirect requests for my_content_type node detail pages to node/123.
   *
   * @param GetResponseEvent $event
   * @return void
   */
  public function redirectMyContentTypeNode(RequestEvent $event) {