How do I enable aggregation in the settings?
Your view appears to be a search API view, for which "aggregation" is not supported. Which is also why the "Use aggregation" option is not visible (credits: comment to question Clive).

A possible solution (work around?) to make the aggregation option available, is to NOT use the Search API for your view, and instead use a regular content type view (credits: comment to question Alfred Armstrong).

PS: Comments may get deleted, for all sorts of reasons, which is why I included them in this answer ...