It works for me. 10x for the solution and respect for the help.
I am witht he last version of Woo commerce 2.3.5 and the last version of the plugin Version 3.6.6 .
I experience the same error by finalizing an order:
Fatal error: Call to undefined method WooCommerce::add_error() in wp-content/plugins/woocommerce-checkout-manager/woocommerce-checkout-manager.php on line 868
