1. Enable modules (including REST UI):

[![enter image description here][1]][1]

2. Enable the Content in REST resources:

[![enter image description here][2]][2]

3. Give permissions on `/admin/people/permissions#module-rest` ??:

[![enter image description here][3]][3]

4. Request returns `403 Forbidden` error:

[![enter image description here][4]][4]

Reference: 
https://drupal.stackexchange.com/questions/166728/getting-a-403-error-in-rest#

  [1]: https://i.sstatic.net/eZpyE.png
  [2]: https://i.sstatic.net/UvFbV.png
  [3]: https://i.sstatic.net/yh3fk.png
  [4]: https://i.sstatic.net/HYh5r.png