This error occurs when you are trying to access something but you are not authorised to do. The reasons are given below.
The reason for getting this error is because the registration has been disabled by the administrator. Starting from Joomla 3.4, registration is disabled by default.
Visit Joomla! documentation for more information about Joomla User Registration
The reason for is, you access an item (esp. Menu item) but you don't have the right to access it. For example, there is a menu item Register whose access level is Registered. When you access the menu item as guest, you will get this error.