It seems that you added the add to cart button yourself in the description of the product. However, the link of the add to cart button is not valid. It is:
wetick.com/index.php?option=com_hikashop...oduct_id=0&Itemid=54
Where It should be:
wetick.com/index.php?option=com_hikashop...oduct_id=6&Itemid=54
You can correct that by editing the product description.
Normally, the "add to cart" button is handled automatically by HikaShop. However, if the stock of the product is 0, or if the product start sale date is in the future, or if the product end sale date in on the past, or if the "catalogue mode" of the tab display in the configuration is set to on, the "add to cart" button will be hidden from the product page. Moreover, the text of the button can easily be changed by editing the translation file under the tab languages of the configuration. You shoul not have to add that button yourself, although it is possible.