Hi,
Is the page completely blank or only the main area ?
If it's only the main area, then it means that your template is not displaying the messages added by HikaShop.
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" />
Or sometimes, your template has an option to turn that display off. So first, check about such option in your template options screen and turn it on.
If you don't find it, please edit the index.php file from your template and make sure the tag <jdoc:include type="message" /> is added to it.
If not, please add it just before the tag <jdoc:include type="component" />
This tag should already be present in all templates.
If the page is completely blank, it means that there is a fatal error during the processing of that page. In that case, please activate the "error reporting" and "debug mode" settings of the Joomla configuration and try again. You should then get some error message on that blank page which will help us understand what's going on. Also, in that case, please provide the version number of your HikaShop.