Internal Error

Look at the server error log – it will have a more detailed error message.

I checked the server error log
This is what i found

WC_Checkout->posted was called incorrectly. Use $_POST directly. Backtrace: require(‘wp-blog-header.php’), require_once(‘wp-includes/template-loader.php’), do_action(‘template_redirect’), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WC_AJAX::do_wc_ajax, do_action(‘wc_ajax_checkout’), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WC_AJAX::checkout, WC_Checkout->process_checkout, WC_Checkout->create_order, WC_Order->save, WC_Data_Store->create, WC_Order_Data_Store_CPT->create, do_action(‘woocommerce_new_order’), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, AV8_Cart_Reports->save_order_id, AV8_Cart_Receipt->save_order_id, WC_Checkout->__get, wc_doing_it_wrong. This message was added in version 3.0.0.

AV8_Cart_Receipt is triggering a warning but this shouldn’t make an internal error. Were there any fatal errors logged?

Does disabling AV8_Cart_Receipt prevent it?

Fatal Error
PHP Fatal error: Call to a member function get_id() on null in /home/edabbonm/public_html/wp-content/plugins/woocommerce-advanced-notifications/includes/class-wc-advanced-notifications.php on line 165

And these were all the errors triggered for that order

to me

[24-Apr-2017 06:54:15 UTC] WC_Checkout->posted was called incorrectly. Use $_POST directly. Backtrace: require(‘wp-blog-header.php’), require_once(‘wp-includes/template-loader.php’), do_action(‘template_redirect’), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WC_AJAX::do_wc_ajax, do_action(‘wc_ajax_checkout’), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WC_AJAX::checkout, WC_Checkout->process_checkout, WC_Checkout->create_order, WC_Order->save, WC_Data_Store->create, WC_Order_Data_Store_CPT->create, do_action(‘woocommerce_new_order’), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, AV8_Cart_Reports->save_order_id, AV8_Cart_Receipt->save_order_id, WC_Checkout->__get, wc_doing_it_wrong. This message was added in version 3.0.0.
[24-Apr-2017 06:54:17 UTC] status was called incorrectly. Order properties should not be accessed directly. Backtrace: require(‘wp-blog-header.php’), require_once(‘wp-includes/template-loader.php’), do_action(‘template_redirect’), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WC_AJAX::do_wc_ajax, do_action(‘wc_ajax_checkout’), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WC_AJAX::checkout, WC_Checkout->process_checkout, WC_Checkout->process_order_payment, WC_Gateway_COD->process_payment, WC_Order->update_status, WC_Order->save, WC_Order->status_transition, do_action(‘woocommerce_order_status_processing’), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WC_pdf_functions->woocommerce_completed_order_create_invoice, WC_Abstract_Legacy_Order->__get, wc_doing_it_wrong. This message was added in version 3.0.
[24-Apr-2017 06:54:17 UTC] id was called incorrectly. Order properties should not be accessed directly. Backtrace: require(‘wp-blog-header.php’), require_once(‘wp-includes/template-loader.php’), do_action(‘template_redirect’), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WC_AJAX::do_wc_ajax, do_action(‘wc_ajax_checkout’), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WC_AJAX::checkout, WC_Checkout->process_checkout, WC_Checkout->process_order_payment, WC_Gateway_COD->process_payment, WC_Order->update_status, WC_Order->save, WC_Order->status_transition, do_action(‘woocommerce_order_status_pending_to_processing’), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WC_Advanced_Notifications->new_order, wc_get_template, include(‘/plugins/woocommerce-advanced-notifications/templates/emails/new-order.php’), do_action(‘woocommerce_email_after_order_table’), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WooCommerce_Order_Barcodes->get_email_barcode, WC_Abstract_Legacy_Order->__get, wc_doing_it_wrong. This message was added in version 3.0.
[24-Apr-2017 06:54:20 UTC] id was called incorrectly. Order properties should not be accessed directly. Backtrace: require(‘wp-blog-header.php’), require_once(‘wp-includes/template-loader.php’), do_action(‘template_redirect’), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WC_AJAX::do_wc_ajax, do_action(‘wc_ajax_checkout’), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WC_AJAX::checkout, WC_Checkout->process_checkout, WC_Checkout->process_order_payment, WC_Gateway_COD->process_payment, WC_Order->update_status, WC_Order->save, WC_Order->status_transition, do_action(‘woocommerce_order_status_pending_to_processing’), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WC_Advanced_Notifications->new_order, wc_get_template, include(‘/plugins/woocommerce-advanced-notifications/templates/emails/new-order.php’), do_action(‘woocommerce_email_after_order_table’), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WooCommerce_Order_Barcodes->get_email_barcode, WC_Abstract_Legacy_Order->__get, wc_doing_it_wrong. This message was added in version 3.0.
[24-Apr-2017 06:54:20 UTC] status was called incorrectly. Order properties should not be accessed directly. Backtrace: require(‘wp-blog-header.php’), require_once(‘wp-includes/template-loader.php’), do_action(‘template_redirect’), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WC_AJAX::do_wc_ajax, do_action(‘wc_ajax_checkout’), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WC_AJAX::checkout, WC_Checkout->process_checkout, WC_Checkout->process_order_payment, WC_Gateway_COD->process_payment, WC_Order->update_status, WC_Order->save, WC_Order->status_transition, do_action(‘woocommerce_order_status_pending_to_processing’), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WC_Emails::send_transactional_email, do_action_ref_array, WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WC_pdf_functions->woocommerce_completed_order_create_invoice, WC_Abstract_Legacy_Order->__get, wc_doing_it_wrong. This message was added in version 3.0.
[24-Apr-2017 06:54:20 UTC] id was called incorrectly. Order properties should not be accessed directly. Backtrace: require(‘wp-blog-header.php’), require_once(‘wp-includes/template-loader.php’), do_action(‘template_redirect’), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WC_AJAX::do_wc_ajax, do_action(‘wc_ajax_checkout’), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WC_AJAX::checkout, WC_Checkout->process_checkout, WC_Checkout->process_order_payment, WC_Gateway_COD->process_payment, WC_Order->update_status, WC_Order->save, WC_Order->status_transition, do_action(‘woocommerce_order_status_pending_to_processing’), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WC_Emails::send_transactional_email, do_action_ref_array, WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WC_Email_New_Order->trigger, WC_Email->get_content, WC_Email_New_Order->get_content_html, wc_get_template_html, wc_get_template, include(‘/plugins/woocommerce/templates/emails/admin-new-order.php’), do_action(‘woocommerce_email_order_details’), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WC_Emails->order_details, wc_get_template, include(‘/plugins/woocommerce/templates/emails/email-order-details.php’), do_action(‘woocommerce_email_after_order_table’), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WooCommerce_Order_Barcodes->get_email_barcode, WC_Abstract_Legacy_Order->__get, wc_doing_it_wrong. This message was added in version 3.0.
[24-Apr-2017 06:54:21 UTC] id was called incorrectly. Order properties should not be accessed directly. Backtrace: require(‘wp-blog-header.php’), require_once(‘wp-includes/template-loader.php’), do_action(‘template_redirect’), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WC_AJAX::do_wc_ajax, do_action(‘wc_ajax_checkout’), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WC_AJAX::checkout, WC_Checkout->process_checkout, WC_Checkout->process_order_payment, WC_Gateway_COD->process_payment, WC_Order->update_status, WC_Order->save, WC_Order->status_transition, do_action(‘woocommerce_order_status_pending_to_processing’), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WC_Emails::send_transactional_email, do_action_ref_array, WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WC_Email_Customer_Processing_Order->trigger, WC_Email->get_content, WC_Email_Customer_Processing_Order->get_content_html, wc_get_template_html, wc_get_template, include(‘/plugins/woocommerce/templates/emails/customer-processing-order.php’), do_action(‘woocommerce_email_order_details’), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WC_Emails->order_details, wc_get_template, include(‘/plugins/woocommerce/templates/emails/email-order-details.php’), do_action(‘woocommerce_email_after_order_table’), WP_Hook->do_action, WP_Hook->apply_filters, call_user_func_array, WooCommerce_Order_Barcodes->get_email_barcode, WC_Abstract_Legacy_Order->__get, wc_doing_it_wrong. This message was added in version 3.0.
[24-Apr-2017 06:54:21 UTC] PHP Fatal error: Call to a member function get_id() on null in /home/edabbonm/public_html/wp-content/plugins/woocommerce-advanced-notifications/includes/class-wc-advanced-notifications.php on line 165

Fatal Error :
PHP Fatal error: Call to a member function meta_exists() on null in /home/edabbonm/public_html/wp-content/plugins/woocommerce/includes/class-wc-checkout.php on line 996