Hello,
1. You didn't had any error message because of your "gk_shop_and_buy" template, if I test it with the "Beez3" template, I correctly have a "The payment method could not be found" error message.
Regarding your template issue, you'll probably find the solution
there
:
HikaShop uses the core Joomla messaging system to display messages on your website so if you don't see any message when you don't enter correct information during the checkout process, that's probably because your Joomla template does not contain the tag to display Joomla messages: <jdoc:include type="message" />
2. Thanks to your feedback I was able to fix it on your website and for the next Hikashop version.
So I just added a fix on your website and it should work fine now.