The Auto Sync feature for portals has been added in our ORD release (July 2016).  If you have created a Portal in a prior version, it is not possible to add this new Auto Sync feature to your existing portal (i.e. this feature is not backward compatible).  To enable Auto Sync, you will need to create a new publishing portal (see Creating Publishing Portals) with the same attributes and content as your existing portal - please note that this will require a new URL.