The error (An Error occurred when creating the shipment: Authentication failed) from Parcelforce typically occurs due to incorrect or outdated login credentials, often caused by a recent password or account detail change.
To resolve this issue please review the steps below:
Confirm that the Parcelforce username and password stored in Mintsoft are correct.
If the password has recently changed, update the login details in the courier integration settings.
By following these steps, you should be able to resolve the error.