"Add product" or "Add blank line" get errors in /admin/store/orders/XX/edit

in page, /admin/store/orders/XX/edit,
when I click the button "Add product" or "Add blank line", I get errors as following:
03-Feb-2017 10:24:58 America/Toronto] PHP Fatal error: Call to undefined function Drupal\uc_order\Plugin\Ubercart\OrderPane\uc_order_pane_admin_comments() in /home/XXXXXXX/public_html/example.com/modules/ubercart/uc_order/src/Plugin/Ubercart/OrderPane/Products.php on line 566