Fatal error: Uncaught Error: Interface "Magento\Framework\Event\ObserverInterface" not found in /home/airtimer/public_html/vendor/magento/module-quote/Observer/SubmitObserver.php:18 Stack trace: #0 {main} thrown in /home/airtimer/public_html/vendor/magento/module-quote/Observer/SubmitObserver.php on line 18