Fatal error: Uncaught Error: Call to undefined method WooCommerce::add_error() in /www/htdocs/w01417b1/schuelerplaner.xcentric.de/wp-content/plugins/quantities-and-units-for-woocommerce/includes/class-wcqu-validations.php:105 Stack trace: #0 /www/htdocs/w01417b1/schuelerplaner.xcentric.de/wp-content/plugins/quantities-and-units-for-woocommerce/includes/class-wcqu-validations.php(31): WC_Quantities_and_Units_Quantity_Validations->validate_single_product(true, 21721, 1, false, NULL, NULL) #1 /www/htdocs/w01417b1/schuelerplaner.xcentric.de/wp-includes/class-wp-hook.php(286): WC_Quantities_and_Units_Quantity_Validations->add_to_cart_validation(true, 21721, 1) #2 /www/htdocs/w01417b1/schuelerplaner.xcentric.de/wp-includes/plugin.php(208): WP_Hook->apply_filters(true, Array) #3 /www/htdocs/w01417b1/schuelerplaner.xcentric.de/wp-content/plugins/woocommerce_neu/includes/class-wc-form-handler.php(776): apply_filters('woocommerce_add...', true, 21721, 1) #4 /www/htdocs/w01417b1/schuelerplaner.xcentric.de/wp-content/plugins/woocomme in /www/htdocs/w01417b1/schuelerplaner.xcentric.de/wp-content/plugins/quantities-and-units-for-woocommerce/includes/class-wcqu-validations.php on line 105