WordPress Categories

Hi. if you go to my website, https://www.teenissues.net, I can’t seem to figure out why the link to the categories go off to an error page.

I have created 3 “test posts” and categorized each one. I believe two of them are under “introduction” and the other one is under ‘Parents’. However, I want it so that when someone clicks on the category “Introduction”, the two posts that I have categorized is visible.

Can someone show me how to do this please?

Thanks.