Anyone ever run into a problem where their field sets do not show up when editing / creating a new

Merchandising Rule Set?

It only shows for the example / demo site. As soon as I select the current site, all of the field sets in the dropdown go missing. It feels like I am missing one step in associating field sets with a site somehow?

this is a really old bug. The workaround is to make the website internal name match the external name (in setup website)

You can make the name match as Pablo says, set up the rule, then revert the website internal name if you want

Thanks! Will give it a try