Hi,
That's the correct code in the correct place.
However, this code change will only work if you have the "checkout legacy" setting activated and use the old checkout system of HikaShop.
The new checkout system uses other view files as it works completely differently from the old checkout.
With the new checkout, you'll want to edit the file checkout / show.php via the menu Display>Views and add such code at the top:
$cart = $this->checkoutHelper->getCart();
$total = 0;
foreach($cart->products as $i => $row){ $total +=$row->cart_product_quantity; }
if($total%6){
$app = JFactory::getApplication();
$app->enqueueMessage('Please order in multiples of 6 bottles');
return;
}
Note that this will hide the checkout interface when the quantity is not correct.