You will encounter this error if you have checked the option to allow (the page) in MasteryPaths. 

When a page is allowed in MasteryPaths, the only way for a student to view the page is via a MasteryPath.

Therefore, in Student View, this page does not appear in the list of pages, and in Student View on the page itself, an Access denied error is shown.

Please refer to the last point in the notes section here.