Error loading navigation: The Managed Navigation term set is improperly attached to the site

Solution :

Add the at-least one Navigation of Term-driven pages section to the Term Store Management section

Refresh the page, the issue will be resolved.

Here is the link how to manage SharePoint 2013 Navigation using Friendly URLs

https://www.nothingbutsharepoint.com/sites/eusp/Pages/SharePoint-2013-Navigation-using-Managed-Metadata-Term-Store.aspx