ParcelForce Authentication error occurs due to invalid/expired ParcelForce credentials on EasyPost / ParcelForce account.
You will need to reach out to [email protected] to obtain your production credentials for ParcelForce, then go to https://www.easypost.com/account/carriers and add the updated information to your ParcelForce account.
When you go to your carrier account at https://www.easypost.com/account/carriers - click on EDIT next to your ParcelForce account. You will see this:
In the center, under "Production Credentials" :
Try entering a number "1" and try buying your shipment again and see if it will work. If not, try inputting a number "2" instead, and click UPDATE to complete the EDIT.
For further assistance, contact Easypost at [email protected].