Step 1: Log into your Drupal 8 admin interface.
Step 2: Look to the top menu bar on the page and click on the Menu tab.
Step 3: A second menu bar will appear. Click on the People option to continue.
Step 4: Now we are on the People where it lists the site members. Click on the Permissions tab in the upper right corner.
Step 5: 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.
Step 6: 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.