To fix this error, you will need to disable a setting called "Future Shipping Dates", in your FedEx Settings.
First, navigate to the sidebar in Shiptheory, select carriers and then click on your FedEx integration.
Once you are in your FedEx Carrier Settings page, scroll to the bottom and select the "Advanced Options" tab. Scroll down until you see an option called "Enable Future Shipping Dates" and change this to "No"
Once this has been done, save your changes and this error will no longer occur when booking consignments. You can now retry any affected shipment!
If you have tried this solution and the error persists, please don't hesitate to get in contact with us either at support@shiptheory.com or call us on 0117 403 4313!