You must configure at the product level, to display the Pre-Order option on the product detail page. For that navigate to Shopify Admin and click on the Product.
By navigating to the product details, you will find a checkbox as Continue selling when out of stock it means to continue the product selling even after the product is out of stock.
You must enable this option to display the Pre-Order option to the store as per configuration from the Commerce and- Pre-Order app.