When a reader clicks on a category in the table of contents, the default behavior is for the category to toggle open and closed to show and hide its content. This is called progressive disclosure in user experience, and helps maintain focus by reduce clutter, confusion, and cognitive workload.
If you would like clicking the category to navigate to the category landing page, start by editing the category by clicking on the 'gear' Edit icon next to it as shown below:
Next, choose Navigate under the Table of Contents Options on the category's edit page:
You will also find the option to hide the contents of the category completely in the same section.