Gettig error message "Unauthorized access"

[dev_ashish94_17]

I got a below message.
You are not authorized to view this page. The transaction has not been processed.

Error Reference Number: E-1B8D2CF9958E44A197CFF163BC1871F8

Find my code below:

var model = new CheckoutModel

            {

                AccessKey = "XXXXXXXXXXXXXXXXXX",

                ProfileId = "XXXXXXXXXXXXXXXXXXXX",

                TransactionUUID = Guid.NewGuid().ToString(),

                SignedDateTime = DateTime.UtcNow.ToString("yyyy-MM-ddTHH:mm:ssZ"),

                Locale = "en-us",

                TransactionType = "sale",

                Amount = "100.00",

                Currency = "USD",

                ReferenceNumber = Guid.NewGuid().ToString().Replace("-", "").Substring(0, 12),

                // Example billing info

                CustomerEmail = "john.doe@example.com",

                BillingFirstName = "John",

                BillingLastName = "Doe",

                BillingAddress = "123 Main St",

                BillingCity = "Los Angeles",

                BillingState = "CA",

                BillingPostalCode = "90001",

                BillingCountry = "US",

                ReturnUrl = baseUrl + AppConstants.ReturnURL.ToString(),

                CancelUrl = baseUrl + AppConstants.CancelURL.ToString(),

            };

            var signedFields = new[]

            {

            "access_key", "profile_id", "transaction_uuid", "signed_field_names", "unsigned_field_names",

            "signed_date_time", "locale", "transaction_type", "amount", "currency", "reference_number",

            "customer_email", "bill_to_forename", "bill_to_surname", "bill_to_email",

            "bill_to_address_line1", "bill_to_address_city", "bill_to_address_state",

            "bill_to_address_postal_code", "bill_to_address_country"

        };

            model.SignedFieldNames = string.Join(",", signedFields);

            model.UnsignedFieldNames = "";

            model.Signature = GenerateSignature(model, signedFields);

private bool VerifySignature(System.Collections.Specialized.NameValueCollection form, string signedFieldNames, string receivedSignature)

        {

            var signedFields = signedFieldNames.Split(',');

            var keyValuePairs = new List<string>();

            foreach (var field in signedFields)

            {

                keyValuePairs.Add($"{field}={form[field]}");

            }

            string dataToSign = string.Join(",", keyValuePairs);

            string secretKey = ConfigurationManager.AppSettings["CyberSource_SecretKey"];

            using (var hmac = new System.Security.Cryptography.HMACSHA256(Encoding.UTF8.GetBytes(secretKey)))

            {

                var computedSignature = Convert.ToBase64String(hmac.ComputeHash(Encoding.UTF8.GetBytes(dataToSign)));

                return computedSignature == receivedSignature;

            }

        }

        private string GenerateSignature(CheckoutModel model, string[] signedFields)

        {

            var keyValuePairs = new List<string>();

            foreach (var field in signedFields)

            {

                string value = GetFieldValue(model, field);

                keyValuePairs.Add($"{field}={value}");

            }

            string dataToSign = string.Join(",", keyValuePairs);

            string secretKey = ConfigurationManager.AppSettings["CyberSource_SecretKey"];

            byte[] keyBytes = Encoding.UTF8.GetBytes(secretKey);

            byte[] dataBytes = Encoding.UTF8.GetBytes(dataToSign);

            using (var hmac = new HMACSHA256(keyBytes))

            {

                return Convert.ToBase64String(hmac.ComputeHash(dataBytes));

            }

        }

        private string GetFieldValue(CheckoutModel model, string fieldName)

        {

            // Map CyberSource form field names to model properties

            switch (fieldName)

            {

                case "access_key": return model.AccessKey;

                case "profile_id": return model.ProfileId;

                case "transaction_uuid": return model.TransactionUUID;

                case "signed_field_names": return model.SignedFieldNames;

                case "unsigned_field_names": return model.UnsignedFieldNames;

                case "signed_date_time": return model.SignedDateTime;

                case "locale": return model.Locale;

                case "transaction_type": return model.TransactionType;

                case "amount": return model.Amount;

                case "currency": return model.Currency;

                case "reference_number": return model.ReferenceNumber;

                case "customer_email":

                case "bill_to_email": return model.CustomerEmail;

                case "bill_to_forename": return model.BillingFirstName;

                case "bill_to_surname": return model.BillingLastName;

                case "bill_to_address_line1": return model.BillingAddress;

                case "bill_to_address_city": return model.BillingCity;

                case "bill_to_address_state": return model.BillingState;

                case "bill_to_address_postal_code": return model.BillingPostalCode;

                case "bill_to_address_country": return model.BillingCountry;

                default: return "";

            }

        }

@model Ecommerce.ViewModels.CheckoutModel

@{

    Layout = null;

}

    <h2>Redirecting to Secure Checkout...</h2>

    <p>Please wait while we redirect you to the payment gateway...</p>

    <form id="cybersourceForm" method="post" action="https://testsecureacceptance.cybersource.com/pay">

        @Html.Hidden("access_key", Model.AccessKey)

        @Html.Hidden("profile_id", Model.ProfileId)

        @Html.Hidden("transaction_uuid", Model.TransactionUUID)

        @Html.Hidden("signed_field_names", Model.SignedFieldNames)

        @Html.Hidden("unsigned_field_names", Model.UnsignedFieldNames)

        @Html.Hidden("signed_date_time", Model.SignedDateTime)

        @Html.Hidden("locale", Model.Locale)

        @Html.Hidden("transaction_type", Model.TransactionType)

        @Html.Hidden("amount", Model.Amount)

        @Html.Hidden("currency", Model.Currency)

        @Html.Hidden("reference_number", Model.ReferenceNumber)

        @Html.Hidden("signature", Model.Signature)

        @Html.Hidden("override_custom_receipt_page", Model.ReturnUrl)

        @Html.Hidden("override_custom_cancel_page", Model.CancelUrl)

        <!-- Customer and billing fields -->

        @Html.Hidden("customer_email", Model.CustomerEmail)

        @Html.Hidden("bill_to_forename", Model.BillingFirstName)

        @Html.Hidden("bill_to_surname", Model.BillingLastName)

        @Html.Hidden("bill_to_email", Model.CustomerEmail)

        @Html.Hidden("bill_to_address_line1", Model.BillingAddress)

        @Html.Hidden("bill_to_address_city", Model.BillingCity)

        @Html.Hidden("bill_to_address_state", Model.BillingState)

        @Html.Hidden("bill_to_address_postal_code", Model.BillingPostalCode)

        @Html.Hidden("bill_to_address_country", Model.BillingCountry)

        <!-- Optional: In case JavaScript is disabled -->

        <noscript>

            <input type="submit" value="Click here to proceed if not redirected automatically" />

        </noscript>

      </form>

<script type="text/javascript">

        window.onload = function () {

            document.forms[0].submit();

        };

    </script>