To add a Cluster, you need to have a least one Measure and one Dimension in the View. To add a Cluster, double-click on the option or drag and drop it on the View.
When you add a Cluster, Tableau opens a window where you can add or remove variables (Measure or Dimension) and define the number of Clusters. Then, Tableau adds a new generated pill, Clusters, on Color. You can use this generated pill wherever you want in the View (to another property or in filters for example) or drag and drop it among the fields in your Data Source to add as a new field.
Here's an example of four clusters on the sub-categories by Sales and Profit:
