If the assignment, quiz or other tool is within a module, please publish the module, if it is not published.
Click the crossed-out icon to publish the module. When published, it will have a tick.
Also please check the Available from date, if applicable.
Other possibilities:
- The student trying to access has Guest Student role. Guest Students cannot access graded assignments and quizzes.
- The course has not reached its open date or is past its end date.