我已经使用了下面的模型，我能够解码模型。

struct APIData: Codable {
    let additionalData: AdditionalData?

    enum CodingKeys: String, CodingKey {
        case additionalData = "additional_data"
    }
}

struct AdditionalData : Codable {

    let adminDiscount : Int?
    let bankname : String?
    let banktxnid : String?
    let bookingId : Int?
    let ccavenueCharge : Int?
    let ccavenueGst : Int?
    let checklistDone : String?
    let createdAt : String?
    let discount : Int?
    let discountCode : String?
    let estimatedPrice : Int?
    let gateway : String?
    let id : Int?
    let mechanicProfit : String?
    let paymentMethod : String?
    let paymentResponse : String?
    let paymentmode : String?
    let status : String?
    let totalPrice : String?
    let txndate : String?
    let txnid : String?
    let updatedAt : String?
    let workPrice : String?
    let workWithPrice : String?

    enum CodingKeys: String, CodingKey {
        case adminDiscount = "admin_discount"
        case bankname = "bankname"
        case banktxnid = "banktxnid"
        case bookingId = "booking_id"
        case ccavenueCharge = "ccavenue_charge"
        case ccavenueGst = "ccavenue_gst"
        case checklistDone = "checklist_done"
        case createdAt = "created_at"
        case discount = "discount"
        case discountCode = "discount_code"
        case estimatedPrice = "estimated_price"
        case gateway = "gateway"
        case id = "id"
        case mechanicProfit = "mechanic_profit"
        case paymentMethod = "payment_method"
        case paymentResponse = "payment_response"
        case paymentmode = "paymentmode"
        case status = "status"
        case totalPrice = "total_price"
        case txndate = "txndate"
        case txnid = "txnid"
        case updatedAt = "updated_at"
        case workPrice = "work_price"
        case workWithPrice = "work_with_price"
    }
    init(from decoder: Decoder) throws {
        let values = try decoder.container(keyedBy: CodingKeys.self)
        adminDiscount = try values.decodeIfPresent(Int.self, forKey: .adminDiscount)
        bankname = try values.decodeIfPresent(String.self, forKey: .bankname)
        banktxnid = try values.decodeIfPresent(String.self, forKey: .banktxnid)
        bookingId = try values.decodeIfPresent(Int.self, forKey: .bookingId)
        ccavenueCharge = try values.decodeIfPresent(Int.self, forKey: .ccavenueCharge)
        ccavenueGst = try values.decodeIfPresent(Int.self, forKey: .ccavenueGst)
        checklistDone = try values.decodeIfPresent(String.self, forKey: .checklistDone)
        createdAt = try values.decodeIfPresent(String.self, forKey: .createdAt)
        discount = try values.decodeIfPresent(Int.self, forKey: .discount)
        discountCode = try values.decodeIfPresent(String.self, forKey: .discountCode)
        estimatedPrice = try values.decodeIfPresent(Int.self, forKey: .estimatedPrice)
        gateway = try values.decodeIfPresent(String.self, forKey: .gateway)
        id = try values.decodeIfPresent(Int.self, forKey: .id)
        mechanicProfit = try values.decodeIfPresent(String.self, forKey: .mechanicProfit)
        paymentMethod = try values.decodeIfPresent(String.self, forKey: .paymentMethod)
        paymentResponse = try values.decodeIfPresent(String.self, forKey: .paymentResponse)
        paymentmode = try values.decodeIfPresent(String.self, forKey: .paymentmode)
        status = try values.decodeIfPresent(String.self, forKey: .status)
        totalPrice = try values.decodeIfPresent(String.self, forKey: .totalPrice)
        txndate = try values.decodeIfPresent(String.self, forKey: .txndate)
        txnid = try values.decodeIfPresent(String.self, forKey: .txnid)
        updatedAt = try values.decodeIfPresent(String.self, forKey: .updatedAt)
        workPrice = try values.decodeIfPresent(String.self, forKey: .workPrice)
        workWithPrice = try values.decodeIfPresent(String.self, forKey: .workWithPrice)
    }

}

你解码的时候根对象应该是APIData，因为你上面AdditionalData的响应里面有一个根对象，见下面代码。

let jsonDecoder = JSONDecoder()
        do{
            let response = try jsonDecoder.decode(APIData.self, from: data)
            completion(response, nil)
        }catch(let error){
            completion(nil,error)
        }