Our Drupal expert helps you out: Drupal 8 Where is my Block?
You create a block programmatically in Drupal 8, then you go to the Block Layout page, expecting that your block will appear on that page, at the bottom, in the Disabled section. This was the behaviour in Drupal 7, but not so in Drupal 8. To see and add your block to the region, you must click on the 'Place block" button of the desired region.
If you did all your coding correctly you will see your block appearing in the model, with all the other available blocks.