Like assignment submissions, assignment reviews are optional. You will only be able to review an assignment if you have submitted one of your own.
If you've submitted an assignment, we recommend you review at least one assignment from your peers. Some courses may recommend reviewing more than one - where this is the case, it should be clearly stated in the assignment review step.
Whenever you visit the assignment review page, you will see a new assignment for review. When you're satisfied with the number of reviews you've submitted, you can move on using the forward arrow at the bottom of the step.