Hi @thvi try this. When you add “The Loop” to the part of the page you want to display your post enter a name for query into the “Custom WP Query Name” section, see screenshot (I used stuff_category_query but enter whatever you feel is appropriate).
Then in the Category Parameters section further down enter the name of the category you want to display (in my case the category I wanted to display was called “stuff”). At that stage I just saved/exported and it displayed only my chosen category posts.
Hope this works for you, let us know if you have any issues and we’ll try to help.