Fatal error: Uncaught Error: Class "Drupal\Core\Form\ConfigFormBase" not found in /home/qcsvlel/www/core/lib/Drupal/Core/Installer/Form/SiteConfigureForm.php:20 Stack trace: #0 {main} thrown in /home/qcsvlel/www/core/lib/Drupal/Core/Installer/Form/SiteConfigureForm.php on line 20