Hello,
It looks like you have a view override for "product / quantity" or "product / show_quantity" with a JS error.
HikaShop 3 do not use anymore the function "hikashopCheckQuantityChange" and in your case, the "min" and "max" parameter are not defined while they should be "0".
In the legacy cart system, we already have a patch to force these two parameters as integer if they are empty but in your website, that fixed error is still there.
So it means that you have an override for a old view, or a custom plugin with the error. But that issue is no more possible in HikaShop 3 and the new "add to cart" system.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.