If your pre-order products are showing as sold out during checkout, it could be due to inventory tracking across multiple locations. Specifically, the issue may arise if one of your locations is not set as a fulfillment location.
How to Fix This Issue
To resolve this, you’ll need to either transfer the inventory to a location designated as a fulfillment location or update the current location with inventory to be a fulfillment location.
Steps to Change a Location to a Fulfillment Location:
Go to Shopify Settings and select “Locations” from the left navigation menu.
Choose the location you wish to update.
Scroll down and find the “Fulfillment details” option.
Check the box to designate the location as a fulfillment location.
Scroll back up and save the settings.
Once this is done, the issue should be resolved, and your pre-order products will no longer show as sold out at checkout.