Problem with Amazon SP-API estimateTransport

I am having trouble getting estimateTransport() to work
It is returning
FTXGatewayService.estimateLtlShipment returned error code: 412

Why?

The ShipmentType of Small Parcel / "SP" works, but "LTL" does not.

The Amazon workflow says to first
PUT to fba/inbound/v0/shipments/FBAxxxx/transport
Then
POST to fba/inbound/v0/shipments/FBAxxxx/estimate

Body for the PUT to transport:
{
    "IsPartnered": true,
    "ShipmentType": "LTL",
    "TransportDetails": {
        "PartneredLtlData": {
            "Contact": {
                "Name": "Test1",
                "Phone": "234-343-3434",
                "Email": "[email protected]",
                "Fax": "234-343-3434"
            },
            "BoxCount": 1,
            "SellerFreightClass": "50",
            "FreightReadyDate": "2022-12-25",
            "PalletList": [{
                    "Dimensions": {
                        "Length": 13,
                        "Width": 13,
                        "Height": 13,
                        "Unit": "inches"
                    },
                    "Weight": {
                        "Value": 13,
                        "Unit": "pounds"
                    },
                    "IsStacked": true
                }
            ],
            "TotalWeight": {
                "Value": 13,
                "Unit": "pounds"
            },
            "SellerDeclaredValue": {
                "CurrencyCode": "USD",
                "Value": 20
            }
        }
    }
}