Out-of-stock products are still available for purchase

1. For Shopify:
1. Go to your shop and open default products.
2. Check that the checkmark "continue selling when out of stock" is not checked.
3. If your site has been customized in the past, contact support, if not, continue with the next step.
4. Check if 'synced location’ is turned on, the product may have inventory in one of your other locations.
5. Contact support for help.
2. For BigCommerce:
1. Go to your shop and open a default product.
2. Check under availability and see if you allow pre-orders 

3. If nothing worked, contact support for help.