The error "Unable to complete the order. The [ ] item is out of stock" is displayed when creating an order. What can I do?

Please check what level of data verification for products is selected in your Site Settings. To do this, go to the Site Settings → Payment Systems → General payment settings → Level of data verification from blocks before proceeding to payment → select the "Allow selling products without checks" option.

See the example:

Once you have done that, you need to save the settings and re-publish all pages of your website.