Not able to create Shipping Instructions For Sales Order Using REST API

We are trying to leverage the below REST API to pass shipping instructions at the SO header level and receiving the error "Invalid Attribute(400)

/fscmRestApi/resources/11.13.18.05/salesOrdersForOrderHub

Please find below payload.

{
 "SourceTransactionNumber": "48077",
 "SourceTransactionSystem": "EDI",
 "SourceTransactionId": "000004502541090",
 "RequestingBusinessUnitName": "PET US BU",
 "BusinessUnitName": "PET US BU",
 "BuyingPartyNumber": "10191",
 "CustomerPONumber": "48077",
 "TransactionalCurrencyCode": "USD",
 "CurrencyConversionType": "Corporate",
 "TransactionOn": "2026-06-15T19:00:00-05:00",
 "TransactionTypeCode": "EDI",
 "RequestingLegalEntity": "Simmons Pet Food Inc",
 "OrigSystemDocumentReference": "",
 "PartialShipAllowedFlag": false,
 "PricedOn": "2026-06-15T19:00:00-05:00",
 "FreezePriceFlag": false,
 "FreezeShippingChargeFlag": false,
 "FreezeTaxFlag": false,
 "SubmittedFlag": true,
 "RequestedFulfillmentOrganizationCode": "6330",
 "ShippingCarrier": "F.O.B. Simmons",
 "ShippingServiceLevel": "Ground",
 "ShippingMode": "Truckload",
 "shippingInstructions": "48077/0002295402 APPOINTMENT AT LEAST 3 BUSINESS DAYS PRIOR CALL 2766235000 EXT 5395 CARRIERS LOOKIN",
 "ShipsetFlag": true,