Did you enter a declared value amount? If so, you will get that error with FedEx
Hello @Seller_ogqbG0HlQuYoY
As @Seller_Ej9F0gKcyVb1R wrote, some sellers have reported seeing that error when declaring a value outside of the $101-$499 range.
Declared Value for FedEx in Buy Shipping Not Working
Please let me know if the above applies to your shipment. Despite the above, the purchase should not be returning errors.
I am getting this same error on any shipment option that is not USPS. We will not ship with USPS because of how often they lose shipments. Amazon customer service has largely been unhelpful and won't provide any protection if we ship outside of Amazon buy shipping. The customer service on our case for this so far has been terrible. We currently have no workaround but have been experiencing this issue for 10 days.