Hi,
Like all standard Joomla components, HikaShop integrates with the Joomla Model View Controller pattern.
Thanks to this users can, among other things, easily customize the views of components by creating override files of the views in the template.
So, for example, the file components/com_hikashop/views/product/tmpl/add_to_cart_ajax.php can be overridden by making a copy of that file to templates/yootheme/html/com_hikashop/product/add_to_cart_ajax.php and then customizing that copy.
In HikaShop, we provide the interface in the menu Display>Views, in order to be able to create overrides more easily on your end.
The advantage of doing this is that when you update HikaShop later on, you keep your modifications instead of having to redo them after each HikaShop update. The disadvantage of this is that you keep using the code from when you made the override file. If it was done 5 years ago, then your override file is based on code that was in HikaShop 5 years ago.
And thus, when you update Joomla and Joomla has changed things up in a way that makes the old code generate errors, you won't get the modifications from the original view file in your override.
And you get errors like you reported to us.
Also, some templates ship with view overrides pre created for HikaShop.
So basically, the error either come from your template provider not updating the view overrides they include by default in their template, or from view overrides you (or someone on your end) created in the past.
Either way, these override files are not in HikaShop by default, and we don't know how you or your template provider modified these files in the past, so we can't provide a proper solution for these errors.
Since you reported the issues, I supposed these were not created on your end, and thus must have come from your template provider. But maybe you forgot you had made them in the past ?