Hi,
The solution above is not possible anymore. This was with the old checkout system which was removed in HikaShop 4.0.0
Warehouses won't allow you to restrict the products in the cart.
If you want that the customers can't add another product in the cart when they add that product, I think you could use the limits system.
For example, you could set a weight of 99 999 kg for that product. Then, create a limit ( in the menu Products>Limits ) restricted to the "cart" with a max weight of 99 999 kg.
That way, when other products are in the cart, you won't be able to add this product to the cart. And when this product is in the cart, you won't be able to add other products as long as they don't have a weight of 0.
Alternatively, it's also possible to develop a custom plugin to prevent the add to cart based on custom conditions, implementing the onBeforeProductQuantityCheck event. We actually recently used that event in plugins/hikashop/shopclosehours.php to allow merchants to prevent users from being able to add some or all products to the cart base on the hour of the day:
www.hikashop.com/home/blog/492-hikashop-4-7-1.html#close