https://developers.facebook.com/blog/post/2018/04/04/facebook-api-platform-product-changes
Access to the Events API is not possible at the moment:
…apps currently accessing Events and Groups APIs will lose access today
Update: After Facebook did a lot of changes, it seems to be possible to get events again. You can get events from Pages you manage easily in dev mode. If you want to get public events, you may want to read the answer in this thread: Facebook pages API: “Page Public Content Access” review screencast