What gives? This situation cannot be simpler and more exasperating for a WordPress Gutenberg noob.
A fresh install and any attempt to add a new Post produces an editor “unexpected error” dialog. Install and activate the Gutenberg plugin (5.3.0) and then Blocks can be added and edited. But there is no Reusable Blocks category for adding a new Block and no provision to save one in the 3-dot menu.
The fresh install is WP 5.1.1 with no plugins on https://caftest.000webhostapp.com. Changing Themes makes no difference.