If you're enrolled as a free audit learner, you won't have access to graded assignments and exams. You can expect to see a message encouraging you to upgrade to the verified track on any page that features a graded assignment.
Comparison: Verified vs Audit track
|What's included?||Verified Track||Audit Track|
|Graded Assessments and Exams||yes||no|
|Practice (Ungraded) Assessments||yes||yes|
You can read more about the difference between the free and paid experiences here.