When trying to import orders you might come across the "Failed to Fast Import: Failed to import carrier settings for carrier XXXX and client XXXX" error.
This error is related to a Client and Billing Account misconfiguration.
If you are trying to import an order for a client which doesn't have the billing account you're trying to use set up you will get this error.
To solve it, add the Billing Account for the Client you're trying to import the order for as detailed in this guide: How to add and set up a Billing Account (General).