When you add a student to your class for the first time, you can send them assignments but you will not be able to view their OneDrive and they will not be able to receive assignment resources until they grant you permission to do so. You can see which students have granted permission by the orange warning icon next to their name.
In order to grant you permissions, they must log into Student Dashboard where they will see this message.
Your students must tick the checkboxes next to their teachers and click on 'Grant access'
You will now be able to view their onedrive for this class, and they will be able to download any resources that you have added as part of an assignment.