You have neglected to add a required use statement at the top of your AppController class. Note how it thinks that your version of beforeRender takes an object of type App\Controller\EventInterface (i.e. in the same namespace as your controller, which is where it assumes things are from by default if you don’t tell it otherwise), while the Cake version takes an object of type Cake\Event\EventInterface. Just put
use Cake\Event\EventInterface;
at the top of your AppController class, and all of these warnings and errors will go away. They have nothing whatsoever to do with the plugin.