Help with fixing broken Caetories link

HI all,

The site I am writing about is:
https://thestockwell.com/

If you’ll click on the right hand side on many of the categories you’ll see that they lead to a broken links page otherwise known as a 404 page. The CSS link does work.

Since I did not do anything to create this, I don’t know how to solve this?

What is the line of code that controls these links and where is it found?

thanks.
A