You navigate to a document library's document settings and do not see the Metadata navigation settings link:
In order to configure the metadata navigation settings on a document library within a site, the Metadata Navigation and Filtering site feature must be activated.
Select Site Settings from the settings (gear) menu. Select Manage Site Features under Site Actions. Find the Metadata Navigation and Filtering feature and click the Activate button:
The Metdadata navigation settings link is now available within the Document Settings of the document library: