How to assign a post to a category/categories in Wordpress?

Question

WordPress

I create a post on the site that is displayed on the main page, I made a menu from the headings, only 5 headings, 4 of them are child ones. In the post settings, I attached this post to all headings. But if I select a category, instead of the site showing this entry, it says that the page was not found! (
What's the problem, comrades?
This site is ithook.ru

Answer 1

Here is what he writes

Answer 2

1. Install Query Monitor .
2. Go to the category page.
3. Look what Query Monitor writes: