Two issues cause a "Locked" or "Authorization Required" error message.
1 - Apex times out when left alone too long. Please refresh or go back to Modules in the left-hand navigation and click on the Apex module again. This will sign the student back in.
2 - Student clicked on a quiz but did not finish the quiz. Once a quiz is started, Apex locks students out of any part of the course until they have completed the quiz. Please either complete the quiz or ask your teacher to reset the quiz.