Hi,
The error appears because the minimum quantity configured in the product is higher than the maximum quantity (15 by default) of the quantity dropdown.
Now, add the line:
$increment = min($increment, $max_quantity);
before the line:
$r = range($min_quantity, $max_quantity, $increment);
in the file /components/com_hikashop/views/product/tmpl/show_quantity.php (two places) and it will avoid the error message, even if the product is not configured properly.
We'll add the modification on our end for the next version of HikaShop.