blocks are null on admin theme
Ok, I made it work by the answer from this post: 
How can I programmatically display a block?

And rendered it by adding this in hook_form_alter:

if ($form_id == 'node_article_edit_form') {
  ... code from the other post here ... 
  $block = $plugin_block-&gt;build();
  $form['checkmark_block'] = $block;
}


And then in the twig file:

{{ form.checkmark_block }}


I find it way to difficult for a small task like this, and annoying to spend so much time on it, but one hopefully learns by it.