Error when buying product

Hi,

That's because of the input buttons for the add to cart of your listing in the description of your product.
The inputs are inside the form of the product page.
So the inputs are used by the add to cart system.
I would recommend to move the</form> tag from the "show" file of the view "product" to before the bottom area div tag of the "show_tabular" view file (you'll have to adjust the HTML closing/opening tags) so that the inputs of your listing in the product description are outside of the form of the product page.

Hi,

This simply means that you had modified directly some core files of HikaShop to be able to avoid the problem. Basically, you (or the person who did the customization on the layout of the product page) did what I said in my previous message, but instead of doing it via view overrides in the template, it did it directly in the core files of HikaShop. And thus, when you update HikaShop, it overwrites the core files of HikaShop and you loose these changes.
So again, the problem is not with the update itself but with the way the product page layout is customized.