Hi,
While setting up a test environment with the template on our end to try to reproduce the issue is possible, it could take a while as we don't have a copy of that template. So we'll first have to request it from JSN. And that doesn't mean we'll be able to reproduce it as something else might be at play here, especially since we don't have the layout mentioned in the error message in the code of HikaShop and thus it can't come from the code in HikaShop.
That's why I suggested an access to your website in order to rapidely look into it. That's the fastest way to go about it, actually.
I've requested a copy of the template, so we'll see, but there is a high chance that I won't be able to reproduce the problem.
What you could try in the mean time is to rename the folder templates/YOUR_TEMPLATE/html/com_hikashop/ via FTP to something else. That will deactivate the overrides of HikaShop from your template and it should hopefully fix the problem.