Getting the error - There was an unexpected error while verifying this challenge. Usually this is due to some pre-existing configuration or code in the challenge Org. We recommend using a new Developer Edition (DE) to check this challenge. If you're using a new DE and seeing this error, please post to the developer forums and reference error id: SYMIWPKN.

Tried with multiple playgrounds as well as personal orgs.

Completed the challenge.  Added the notification (see attached image) in the Wave developer org.  It looks like it did not save the first time.  Added the notification a 2nd time, as shown in screen capture.