The main reason for this, is you having no students in the Class.
Go to; Admin > Classes and locate the class in question. If the class shows in red or orange, this means no students are assigned. (This would be green if students were assigned).
To fix this; go to the Add Student to Class tab (at the top) - click the Year group, then simply tick the classes for that year, tick all the students and click save at the bottom. Your Markbook will now work.
If you check the Class and students are assigned, please report this to us so that we can investigate this further for you by emailing; email@example.com