Drupal 8, by default, does not allow non-registered users to leave comments. If you want to allow visitors to be able to leave comments on your site, follow the instructions below.
How to give visitors commenting privileges
- Log into your Drupal 8 admin interface.
Look to the top menu bar on the page and click on the Menu tab.
A second menu bar will appear. Click on the People option to continue.
Now we are on the People where it lists the site members. Click on the Permissions tab in the upper right corner.
This leads you to the master Permissions list. Look for the Comment section and locate the Post Comments permission. Click on the checkbox for the Anonymous User column.
- Once you have selected the checkbox, click on the Save Permissions button at the bottom. This will enable the non-registered visitor to leave comments on your site. Below you can see a shot of a post both before the permission is active and after.
Before After 
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