The answer
[Display-posts] doesn’t work natively. You have to install a plug-in.
The story
I’m wanting to organize my posts into categories and then have separate pages to show the different categories. The idea is sort of wanting to have mini blogs that can be overlapping based on categories and tags.
This tutorial was where I landed:
https://codecraftwp.com/wordpress-show-specific-category-posts-on-page/
I spent a few minutes and became frustrated until I finally starting thinking that I’m missing something.
Went back to Google and found this:
Of course then I realized that the shortcode had to reference SOMETHING. So I installed the plug-in and now we’re cooking.