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On my Drupal 8 I have a blog, which should have a comment function. I added the comment field in the content type of the blog posts and I also checked the checkbox that the comment input field is shown. But the comment section is not displayed. Even in my template file in my theme_preprocess_node function I could not find the comment field. I also tried it with the Bartik theme, but that also didn't work.
My question now is: How can I display the comments section on my blog articles?
Try check or set this permission 
Debugging is the art of taking away all possible causes ... in this case you want to make sure that the permissions are set correctly. so that for the user id you're using, it's allowed to post comments.
Not sure if it is "the" reason for your issue, but without appropriate permission Drupal seems to work as it should.
Another reason might be that you have "the field deactivated in the manage dispay settings of the related content type" (as per your comment below this answer seems to be your case).
