The error code «unknown order/0/s/» means that you are not sending a “SHASign” in your hidden HTML fields, even though you have entered an additional string in field 3.2 of your technical information page indicating you want to use a SHA signature with each transaction.
For more information on SHA-1, please refer to the Advanced e-Commerce Integration Guide.
See also another explanation here: https://e-payment-postfinance.v-psp.com/en/en/guides/integration%20guides/possible-errors