If your pre-order product is using Default Settings:
In the main menu, click on Default Settings.
Scroll down to the Stock section.
Check the box Enable Inventory Management
Check the box beside Method 2.
Save your changes.
Note: for this feature to work, you must set "Inventory policy" to "Shopify tracks this product's inventory" and set an inventory quantity in your shopify product settings. For Method 2 to work, you must check the box "Allow customers to purchase this product when it's out of stock" in your shopify product settings.
If your pre-order product is using custom settings:
In the main menu, click on Single Products.
Use the search bar to find your pre-order product or scroll down to the Pre-order products table.
Click the pencil icon button in the action column to edit the appropriate product.
Click on a variant to edit it.
Ensure the box Use custom settings is checked.
Scroll down to the Stock section.
Check the box Enable Inventory Management.
Check the box Use Shopify inventory management.
Then check the box beside Method 2.
Save your changes.
Note: for this feature to work, you must set "Inventory policy" to "Shopify tracks this product's inventory" and set an inventory quantity in your Shopify product settings. For Method 2 to work, you must check the box "Allow customers to purchase this product when it's out of stock" in your Shopify product settings.