Then I would suggest you to search this forum for similar cases, for example, the three below point to the same source of problem - that something unexpected had been done, and would it happen to be your case as well? For example:
would you read the notebook again and see if it has asked you not to do something but you have done it?
would there be any notice on the assignment page (before you click into the notebook), or before the assignment page that contains some relevant guidelines?
would you try to refresh the notebook to its original state and then copy the answers back, so as to make sure only answers are added and nothing else in the notebook is changed?
Any other ideas from mentors of this specialization would be very much welcomed!