In the Exercise Enforcing a case processing dependency there is a disconnect between the instructions and the Exercise System link provided.
It is impossible to complete the exercise since the Onboarding Case is completely empty in the environment provided. There is no data model, workflow, views etc. however, the instructions state in Step 3 "
In the Benefits Enrollment stage, click +STEP > More > Utilities > Create Case > Select to create a new child case in the Benefits Enrollment process."
Here is the path to the exercise system contained within the course: