?format=json returns 'Not acceptable format: json'
\Drupal\rest\RestResourceConfigInterface is a namespace not a directory structure.

Configuration files need to be imported into the active configuration which is stored in the database. 

In the case of a single config file:


Go to the
page /admin/config/development/configuration/single/import 
Under "Configuration type" select "REST resource configuration" In the box
At "Paste your configuration here" paste your REST YAML configuration


Configuration can be exported and imported using the config directory, as specified in settings.php with the line $config_directories['sync'] = 'sites/default/config';.  

To export the current active configuration to files in the sync directory use the drush command: drush config-export

To import configuration from files in the sync directory and overwrite your current active configuration, go to the page /admin/config/development/configuration.

Another option is to use the REST UI module to configure your REST services.