How to enable pre-ordering for your store products?
You need to select the checkbox "Allow customers to purchase this product when it's out of stock" for each variant of such products in admin panel of your Shopify store. It can be made on product's variant edit page, as it is shown on this screenshot:
(click to enlarge)
Or it can be made on Inventory page as it is shown on screenshot here:
(click to enlarge)