The Omeka S manual says
Advanced item set search
Site visitors can access an advanced item set search from the Browse Items Sets page, if you have that page accessible. There is a link for Advanced search near the top of the Item Sets page.
On my site I have two item sets. When I display the Item Sets page it shows no such link. What’s up with that? Is it a bug? Does the search link appear only when there are some number of item sets? Someone please elucidate.