If you have visited this help page and have found that there are no links available to you when you go to view an assignment's feedback, there are a couple of things possibly going on.
1. If there is no Grademark link, it may be that the instructor has set up Grademark availability for some future date. That date has not passed and therefore the grademark link is not available.
2. If there is not Similarity Report icon, it is possible that the instructor has not allowed student view of the similarity report.
In both cases, contact the instructor and let him or her know that the icons for accessing Turnitin are not available to students.