When processing orders you might come across the "Hard 190100 - Invalid or Missing ShipmentIdentificationNumber" error.
This error is quite particular because it's a cascading error. This means that other errors end up triggering this one as a final consequence.
The exact problem is that since there are previous problems to be solved, the shipment still cannot generate an identification number.
To solve this issue, solve all the previous errors detailed in the message, and the Hard 190100 error will get resolved automatically as a consequence.