Bear in mind that if you have 'Cluster' available in the mass operations menu in Relativity, it will not function if the admin has not enabled queries for at least one analytics index.
If this is not the case, after selecting a subfolder, (or the top level folder in the browser for all the documents in the workspace) and choosing Cluster, Relativity will present you with a notice reading, "There are currently no indexes with queries enabled available for clustering".
The analytics index must be edited and rerun with the queries enabled. Refer to the console at the right when editing an individual index in the Indexing & Analytics . . . Analytics Index tab.
Queries are enabled after the index is populated and built, but before it is activated.