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Drupal\Component\Plugin\Exception\PluginNotFoundException: The "simple_sitemap_display_extender" plugin does not exist. Valid plugin IDs for Drupal\views\Plugin\ViewsPluginManager are: default in Drupal\Core\Plugin\DefaultPluginManager->doGetDefinition() (line 53 of core\lib\Drupal\Component\Plugin\Discovery\DiscoveryTrait.php).
The error message "The "simple_sitemap_display_extender" plugin does not exist. Valid plugin IDs for Drupal\views\Plugin\ViewsPluginManager are: default" usually occurs when Drupal is unable to find the Views plugin that you are trying to use.
To fix this issue, you can try the following steps:
Make sure that the Simple Sitemap Views module is installed and enabled on your Drupal site. If the module is not installed, you will need to download and install it before the plugin can be used.
Double-check that the plugin ID specified in your Views configuration is correct. In this case, the error message indicates that the plugin ID being used is "simple_sitemap_display_extender", but Views is not able to find a plugin with that ID. Check that the ID matches the ID of the plugin as defined in the Simple Sitemap Views module.
Verify that the plugin file exists in the correct location. For this particular plugin, the file should be located at modules/contrib/simple_sitemap_views/src/Plugin/views/display_extender/SimpleSitemapDisplayExtender.php.
Clear the Drupal cache to ensure that the plugin is properly registered with the system. You can clear the cache by running the following Drush command: drush cache:rebuild
Once you have resolved any issues preventing Drupal from finding the Simple Sitemap Views plugin, the error message should go away and the plugin should be available for use in your Views configuration.