Hi,
I have no reference of "tax_amount" in line 1488 in the product class in HikaShop 2.4.0 ; I just see one single reference in file:
$variant->prices[$k]->taxes[$taxKey]->tax_amount = $tax->tax_amount * $product_price_percentage / 100;
Because it is just a warning (and not an error), you can simply replace the line with
$variant->prices[$k]->taxes[$taxKey]->tax_amount = @$tax->tax_amount * $product_price_percentage / 100;
in order to avoid the warning and put a correct value in the tax_amount variable.
Regards,