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) {