¿Cómo resolver esto mientras el valor de tipo json generado no se puede convertir a system.collections.generic.list? .

Ejecute su código a través de un convertidor de clases JSON y producirá las siguientes estructuras:

Public Class Bd
    Public Property pickupAccountId As String
    Public Property soldToAccountId As String
    Public Property pickupDateTime As DateTime
    Public Property handoverMethod As Integer
    Public Property pickupAddress As PickupAddress
    Public Property shipperAddress As ShipperAddress
    Public Property shipmentItems As List(Of ShipmentItem)
End Class

Public Class ConsigneeAddress
    Public Property name As String
    Public Property address1 As String
    Public Property address2 As String
    Public Property city As String
    Public Property state As String
    Public Property district As String
    Public Property country As String
    Public Property postCode As String
    Public Property phone As String
    Public Property email As String
End Class

Public Class Hdr
    Public Property messageType As String
    Public Property messageDateTime As DateTime
    Public Property accessToken As String
    Public Property messageVersion As String
    Public Property messageLanguage As String
End Class

Public Class ManifestRequest
    Public Property hdr As Hdr
    Public Property bd As Bd
End Class

Public Class PickupAddress
    Public Property name As String
    Public Property address1 As String
    Public Property address2 As String
    Public Property city As String
    Public Property state As String
    Public Property district As String
    Public Property country As String
    Public Property postCode As String
    Public Property phone As String
    Public Property email As String
End Class

Public Class Root
    Public Property manifestRequest As ManifestRequest
End Class

Public Class ShipmentItem
    Public Property consigneeAddress As ConsigneeAddress
    Public Property shipmentID As String
    Public Property packageDesc As String
    Public Property totalWeight As Integer
    Public Property totalWeightUOM As String
    Public Property dimensionUOM As String
    Public Property productCode As String
    Public Property currency As String
    Public Property remarks As String
    Public Property isMult As String
    Public Property valueAddedServices As ValueAddedServices
    Public Property deliveryOption As String
    Public Property shipmentPieces As List(Of ShipmentPiece)
End Class

Public Class ShipmentPiece
    Public Property pieceID As Integer
End Class

Public Class ShipperAddress
    Public Property name As String
    Public Property address1 As String
    Public Property address2 As String
    Public Property city As String
    Public Property state As String
    Public Property district As String
    Public Property country As String
    Public Property postCode As String
    Public Property phone As String
    Public Property email As String
End Class

Public Class ValueAddedService
    Public Property vasCode As String
End Class

Public Class ValueAddedServices
    Public Property valueAddedService As List(Of ValueAddedService)
End Class

Entonces, la deserialización JSON espera devolver una instancia de Root que contiene un ManifestRequest, no una colección, que está contenida en una subclase.

Cuando deserializas JSON, obtienes todos los datos, ¡no solo la parte que podría interesarte!

Está intentando deserializar una única instancia de ValueAddedService y pasarla a la API. Eso estaría bien si estuviera trabajando con una sola instancia, pero en realidad deserializa en una lista, que envía a la API.

[
{
"Version":"1.1",
"TranDtls":{
               "TaxSch":"GST",
               "SupTyp":"B2B",
               "IgstOnIntra":"N",
               "RegRev":"N",
               "EcmGstin":null
},
"DocDtls":{
               "Typ":"INV",
               "No":"1952",
               "Dt":"22/02/2024"
},
"SellerDtls":{
               "Gstin":"29ACUPU9257A1ZT",
               "LglNm":"DORESWAMY VELMURUGAN",
               "TrdNm":"SWARNALAKSHMI",
               "Addr1":"MUTHU COMPLEX, H B ROAD,",
               "Addr2":"HEGGADADEVANAKOTE",
               "Loc":"HEGGADADEVANAKOTE",
               "Pin":571114,
               "Stcd":"29",
               "Ph":"9242889917",
               "Em":"thegreatvelu@gmail.com"
},
"BuyerDtls":{
               "Gstin":"32CEHPB9957C1ZM",
               "LglNm":"GOLDEN INTERLOCK",
               "TrdNm":"GOLDEN INTERLOCK",
               "Pos":"32",
               "Addr1":"1/11, valliyoorkavu",
               "Addr2":"mananthavady",
               "Loc":"mananthavady",
               "Pin":670645,
               "Stcd":"32",
               "Ph":null,
               "Em":null
},
"ValDtls":{
               "AssVal":44687,
               "IgstVal":12512,
               "CgstVal":0,
               "SgstVal":0,
               "CesVal":0,
               "StCesVal":0,
               "Discount":0,
               "OthChrg":0,
               "RndOffAmt":0,
               "TotInvVal":57199
},
                         "ItemList":[{
                                "SlNo":"1",
                                "PrdDesc":"Tyre",
                                "IsServc":"N",
                                "HsnCd":"40112010",
                                "Barcde":null,
                                "Qty":2,
                                "Unit":"NOS",
                                "UnitPrice":22343.50,
                                "TotAmt":44687,
                                "Discount":0,
                                "PreTaxVal":0,
                                "AssAmt":44687,
                                "GstRt":28,
                                "IgstAmt":12512,
                                "CgstAmt":0,
                                "SgstAmt":0,
                                "CesRt":0,
                                "CesAmt":0,
                                "CesNonAdvlAmt":0,
                                "StateCesRt":0,
                                "StateCesAmt":0,
                                "StateCesNonAdvlAmt":0,
                                "OthChrg":0,
                                "TotItemVal":57199
                              }
                              ]
               }
]