If there is not a course assigned to your class via the Class Admin tab, then your Markbook will not load and you may encounter a Whoops error message.
Your Overview tab will look like this, telling you there are no courses assigned:
It is important to make sure that you have a course assigned to your class via the class admin tab. Then, your students will then have access to work on CENTURY. As soon as your students start to complete work on CENTURY, the results will come in via your Markbook.
You can access an article on how to assign courses to classes here.