Default Breadcrumb is not working in bootstrap drupal 8?
The twig code doesn't work and you don't need it for the breadcrumb block to be available in the block layout. It's the other way, first you assign the breadcrumb block in for example the Highlighted region. Then you print the region with the breadcrumbs in page.html.twig:

{{ page.highlighted }}


In D8 it's not possible to access the breadcrumbs in the page template directly. You have to use a Drupal block. See https://www.drupal.org/node/2410773.

BTW Twig Blocks are not connected to Drupal blocks. They are used to extend templates:


  Blocks are used for inheritance and act as placeholders and
  replacements at the same time. They are documented in detail in the
  documentation for the extends tag.