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.