Please make sure that you configured your PostFinance account correctly. If this error happens we are lacking return parameter which should have been returned by PostFinance.
Please make sure that in your PostFinance Configuration > Transaction Feedback the following options are selected
- I would like to receive transaction feedback parameter on the redirection URLs.
The screenshot below shows the referenced setting again. It lists Ingenico however, the settings are also valid for PostFinance, Barclaycard, KBC and Ogone.