Trying to complete this challenge, came accross this error:
Challenge not yet complete... here's what's wrong: 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: MPPOFXLH
To note that I got the error with a trailhead playground, and also with a brand new that I created right after encountering the error.