The magically descriptive error 401 Forbidden "message": "Access denied on creating field 'uid'.".  You probably can sense the love I have for this error.  This was a painful error that took a little while to resolve.  First off I also had this error display as: "Access denied on creating field 'uid' "Access denied on creating field 'name' "Access denied on creating field 'entity_id' And I would have had more if I continued down the resolution path I was on....

In Drupal 8.x hiding the subject line is a quick task. In the admin area of your site go to structure > comment types > {comment type} > manage form display and the URL will be like https://domain.com/admin/structure/comment/manage/{comment}/form-display.  I have put the type in braces as you might have several comment types.  If you only have the default Drupal setting, then use default comments and the path will...