A quick solution to fix this issue is to allow overselling, or to order product when it is out of stock.
- Go to Shopify > Setting > Location
- Click on the Shipping origin address
- Tick on “Fulfill online orders from this location” checkbox. It has to be checked so the inventory can be sold.
Finally, click on Save.
I hope this might be helpful for you. Do not hesitate to contact us for further support.