Fatal error on new version

Our client reported an issue. When they try to change the order status, it causes a fatal error. On my test site I could recreate the issue. I also thought it was odd that it reported the order as cancelled, rather than pending payment. Not sure if that's intended function or if I didn't change a settings. 

the fatal error 



Error Details

=============

An error of type E_ERROR was caused in line 872 of the file /wp-content/plugins/purchase-orders-for-woocommerce/class-purchase-order-gateway.php. Error message: Uncaught Error: Call to undefined method WP_Post::update_meta_data() in /wp-content/plugins/purchase-orders-for-woocommerce/class-purchase-order-gateway.php:872

Stack trace:

#0 /wp-includes/class-wp-hook.php(324): WC_Gateway_Purchase_Order->pofwc_save_purchase_order_meta()

#1 /wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters()

#2 /wp-includes/plugin.php(517): WP_Hook->do_action()

#3 /wp-content/plugins/woocommerce/includes/admin/class-wc-admin-meta-boxes.php(262): do_action()

#4 /wp-includes/class-wp-hook.php(326): WC_Admin_Meta_Boxes->save_meta_boxes()

#5 /wp-includes/class-wp-hook.php(348): WP_Hook->apply_filters()

#6 /wp-includes/plugin.php(517): WP_Hook->do_action()

#7 /wp-includes/post.php(5110): do_action()

#8 /wp-includes/post.php(5212): wp_insert_post()

#9 /wp-admin/includes/post.php(453): wp_update_post()

#10 /wp-admin/post.php(227): edit_post()

#11 {main}

  thrown