Hi,
The plugin has a setting "Display only when discount is applied". If that setting is turned on, the plugin will only display its message on the product page on the frontend if the product has a discount applied to it. However, the product on your link doesn't have a discount applied to it as far as I can see.
Also, the plugin has a setting "Frontend position". If that setting is set to "No", the message won't be displayed.
Also, if that setting is set to another position, the position needs to be in the product page layout. I can see that the product page layout on your website has been customized. It's possible that the position selected in the plugin might have been deleted from the layout on your end leading to the message not displaying. To make sure it doesn't come from that, you can temporarily switch to the default template of Joomla ( or temporarily rename the folder templates/YOUR_TEMPLATE/html/com_hikashop/product/ via FTP to deactivate the overrides of the product page for your template ) and see if it displays or not.
