I’m doing the “Control Access to Objects” challenge in Data Security module. The challenge requires Position object and Recruiters permission set group, but my Trailhead playground doesn’t contain Position, Candidate, Job Application, or Review objects.

You can try refreshing the Trailhead Playground or creating a new Playground and opening the challenge there. Some Trailhead challenges depend on specific sample objects or package metadata that may not be available in every Playground. 

Also, check Setup → Object Manager and search for the required objects. If they’re still missing, it’s likely a Playground configuration issue rather than something you’re doing incorrectly.