How do I fix a "Mismatched entity and/or field definitions" error on my configuration?
You can solve this issue by two ways 
If you have Drush -

drush entity-updates


If you don't use Drush add following script before $response-&gt;send(); in index.php file(located in Drupal root folder) and reload the web page.

try {
  \Drupal::entityDefinitionUpdateManager()-&gt;applyUpdates();
}
catch (EntityStorageException $e) {
  print_r($e);
}


Then remove this code from index.php file.