This error triggers when you are trying to book a shipment to country code GB without filling the following fields in the booking:

SEND address - VAT number
SEND address - EORI number
RECV address - VAT number

RECV address - EORI number

It's these numbers that the carrier needs to facilitate a successful supply chain through the UK customs department. 

For clarification's sake, the carrier has told us that the numbers should correspond with the following:

SEND VAT number -> the VAT number of the shipper
SEND EORI number -> the EORI number of the shipper
RECV VAT number -> the UK VAT number of your entity for importing into the UK (you need to register this number in the UK first to do business there)

RECV EORI number -> the UK EORI number of your entity for importing into the UK (you need to register this number in the UK first to do business there)

Fill the empty fields with the correct numbers to rebook the shipment successfully,