Problem with showing custom html.twig content
I've tried this on my site and used main navigation and it worked. block--mainnavigation.html.twig

What you can check:


Of course check if you have some typo in file name, or try with
default navigation to see if it works.
https://www.drupal.org/node/2598914 Try to disable Twig cache while you are developing.
Try to use cache rebuild with Drush https://drushcommands.com/drush-8x/cache/cache-rebuild/


I don't think that you are doing something wrong, problem is with caching I believe.