why getEditableConfigNames

Mutable or Immutable has nothing to do with config schema, that's something else.

getEditableConfigNames() only works if you actually implement \Drupal\Core\Form\ConfigFormBase and $this->config().

Otherwise you need to explicitly get the config.factory service or \Drupal::configFactory() and then call getEditable() on that, instead of get().

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https://drupal.stackexchange.com/questions/233361/creating-config-schema-file-for-mutable-data