This error can occur under one of two different scenarios:
If you are attempting to access your course on the same day of registration/enrollment. Your enrollment will be active the morning after you have made a full payment.
You click on the EMAIL tool within your course and you receive "Error: Not authorized You are not authorized to view the page you are trying to reach" This is an indication that you have a quiz (or several quizzes) in progress. All quizzes must be completed before the email tool is enabled. This is to prevent the student from referring back to information in their email for assistance during the quiz.