This error will be caused by an issue with the account number for UK Mail/DHL Parcel UK filled out in Shiptheory. 

It's worth making sure that the account number is definitely correct, and that there are no spaces before or after the account number. One more thing to note is that all of the letters need to be upper case.

If all of this has been done and you are still receiving the error when retrying your shipment, please contact support.