Category listing not working on advanced seach form

Hi, When I test out the sample advanced search form code at https://docs.expressionengine.com/latest/add-ons/search/advanced.html#site the category dropdown won’t show the categories on the front end. The code name='cat_id[]' doesn’t work. It’s on the front end of my site so I want Guests to be able to view the categories should it be a permissions issue? In the form tag I nominated all the categories (parent and child) {exp:search:advanced_form result_page="site/search-results" channel="faculty-bio" category="1|2|3|5|6|7|8|9" form_class="form-group" } but I don’t get the category listing in the dropdown. The channels incidentally list fine with name='channel_id[]'

Help appreciated. I assume its a permissions issue? Thanks