Today, I also met the same problems when I set up a testing environment.
Then I found a solution:
1. create a new app in pusher dashboard
2. check option "Want to request a different region?", and select another cluster
3. select "react" as the frontend, and "node.js" as backend
4. submit, and then you will get app_id, key, secret
5. fill out pusher extension settings with the value you get in step 4
6. keep "cluster" blank
7. save and then you will get push notifications
Hope it helps.
