Using the code from Creating a configuration entity type in Drupal 8 to create a sample module for demonstrating use of configuration entities, I got the following log message when browsing to the path /admin/config/system/example
Drupal\Component\Plugin\Exception\PluginNotFoundException: The "example" entity type does not exist. in Drupal\Core\Entity\EntityTypeManager->getDefinition() (line 133 of /var/www/html/opti/core/lib/Drupal/Core/Entity/EntityTypeManager.php).
This is embarrassing, but I'm going to write it up anyway.
I had copied and pasted the code verbatim from the article into new files using the filenames and paths that were indicated. It should have worked!
It turns out I had forgotten one little thing: at the top of each of the php files, I had neglected to insert
What eventually tipped me off was that my text editor, Sublime Text, was not providing the syntax coloring that it normally did.
Laughing my ass off ...
Creating a configuration entity type in Drupal 8
Configuration Entities in Drupal 8