We have just started to get this error and some orders have not automatically being created in magento2.
Magento Order was not created. Reason: We don't have "#####" in the requested quantity, so we'll back order the remaining 1
I understand what the error means (Not enough items in stock) but we create the product on order, and before this started to happen, it worked fine since it was installed over a year ago.
I assume that after an update there is a setting to allow backorder, but can't locate this setting, so some help on how to set this up correctly would be very welcome.