Issue:

When checkout, I got the error:

Fatal error: Uncaught Error: Call to undefined method Magento\Sales\Api\Data\OrderExtension::setBssCustomfield() in /var/www/html/app/code/Bss/CheckoutCustomField/Model/Observer/Sales/OrderLoadAfter.php:75 Stack trace: #0 /var/www/html/vendor/magento/framework/Event/Invoker/InvokerDefault.php(73): Bss\CheckoutCustomField\Model\Observer\Sales\OrderLoadAfter->execute(Object(Magento\Framework\Event\Observer)) #1 /var/www/html/vendor/magento/framework/Event/Invoker/InvokerDefault.php(61): Magento\Framework\Event\Invoker\InvokerDefault->_callObserverMethod(Object(Bss\CheckoutCustomField\Model\Observer\Sales\OrderLoadAfter), Object(Magento\Framework\Event\Observer)) #2 /var/www/html/vendor/magento/framework/Event/Manager.php(66): Magento\Framework\Event\Invoker\InvokerDefault->dispatch(Array, Object(Magento\Framework\Event\Observer)) #3 /var/www/html/var/generation/Magento/Framework/Event/Manager/Proxy.php(95): Magento\Framework\Event\Manager->dispatch('sales_order_loa...', Array) #4 /var/www/html/vendor/magento/framework/Model in /var/www/html/app/code/Bss/CheckoutCustomField/Model/Observer/Sales/OrderLoadAfter.php on line 75


Solution:  

Could you please remove 2 folders: var/di and var/generation, then flush cache to fix the issue?

If there is any problem, please let us know.