Ah, Spring is in the air. So are arms, as people new to Salesforce throw them up during Trailhead challenges where they can’t seem to get the hands-on part to pass even though they see the result they expect.
The Trailhead modules and Superbadges are so well organized and written, it may seem like there is an instructor reviewing you submissions, but that would not be practical, profitable, or in the spirit of a cloud platform. The scoring is done by automated tests that are checking that things match exactly as the instructions provided.
The most common cause is that the learner has mis-typed a value provided, usually the API name (i.e., my_variable__c). Runner up to this is the experienced user who is new to Trailhead and uses their own naming conventions rather than following the instructions (been there, done that).
The third common cause is that the module content was updated but the test was not (doesn’t happen that often, but you can tell when there are a bunch of questions on the Trailhead Community about the same problem).
© Scott S. Nelson