You are describing 2 different aspects.
a) The listing of all posts in a category. This is already happening with you, e.g. here: https://gucu.me.uk/category/old-testament/11-1-kings/ – the link you showed leads to a 404 page because a category with this slug does not exist.
b) So when you call the detail view of a post you want to show all other posts that are in the same category(ies)? So quasi an overview below similar to the categories?