C# PayPal REST API Checkout with Credit Card

Here is my code for checking out with a credit card. Using PayPal Trans works fine but I am missing something for credit card trans.

HttpContext CurrContext = HttpContext.Current;    
APIContext apiContext = Configuration.GetAPIContext();    
Item item = new Item();
            item.name = _ItemDescription;
            item.currency = "USD";
            item.price = _Amount;
            item.quantity = "1";
            item.sku = _UPC;

            List<Item> itms = new List<Item>();
            itms.Add(item);
            ItemList itemList = new ItemList();
            itemList.items = itms;

            Address billingAddress = new Address();
            billingAddress.city = ciTxBx.Text;
            billingAddress.country_code = "US";
            billingAddress.line1 = ad1TxBx.Text;
            billingAddress.line2 = ad2TxBx.Text;
            billingAddress.postal_code = pcTxBx.Text;
            billingAddress.state = stRcb.SelectedValue.ToString();

            CreditCard crdtCard = new CreditCard();
            crdtCard.billing_address = billingAddress;
            crdtCard.cvv2 = scTxBx.Text;
            crdtCard.expire_month = Convert.ToInt32(emonthTxBx.Text);
            crdtCard.expire_year = Convert.ToInt32(eyearTxBx.Text);
            crdtCard.first_name = ccfnTxBx.Text;
            crdtCard.last_name = cclnTxBx.Text;
            crdtCard.number = ccnTxBx.Text;
            crdtCard.type = ConvertLower(cctRcb.SelectedValue.ToString());

            Details details = new Details();
            details.tax = "0";
            details.shipping = "0";
            details.subtotal = _Amount;

            Amount amont = new Amount();
            amont.currency = "USD";
            amont.total = _Amount;
            amont.details = details;

            Transaction tran = new Transaction();
            tran.amount = amont;
            tran.description = _ItemDescription;
            tran.item_list = itemList;

            List<Transaction> transactions = new List<Transaction>();
            transactions.Add(tran);

            FundingInstrument fundInstrument = new FundingInstrument();
            fundInstrument.credit_card = crdtCard;

            List<FundingInstrument> fundingInstrumentList = new List<FundingInstrument>();
            fundingInstrumentList.Add(fundInstrument);

            PayerInfo pi = new PayerInfo();
            pi.email = emTxBx.Text;
            pi.first_name = fnTxBx.Text;
            pi.last_name = lnTxBx.Text;
            pi.shipping_address = billingAddress;

            Payer payr = new Payer();
            payr.funding_instruments = fundingInstrumentList;
            payr.payment_method = "credit_card";
            payr.payer_info = pi;

            Payment paymnt = new Payment();
            paymnt.intent = "sale";
            paymnt.payer = payr;
            paymnt.transactions = transactions;
            try
            {
                Payment createdPayment = paymnt.Create(apiContext);
                CurrContext.Items.Add("ResponseJson", JObject.Parse(createdPayment.ConvertToJson()).ToString(Formatting.Indented));
            }
            catch (PayPal.Exception.PayPalException ex)
            {
                if (ex.InnerException is PayPal.Exception.ConnectionException)
                {
                    CurrContext.Response.Write(((PayPal.Exception.ConnectionException)ex.InnerException).Response);
                }
                else
                {
                    CurrContext.Response.Write(ex.Message);
                }
            }
            CurrContext.Items.Add("RequestJson", JObject.Parse(paymnt.ConvertToJson()).ToString(Formatting.Indented));

Anyone know why I am receiving a "No payload for this request" message?

Well after mocking your code (and hard-coding values), the line that I commented on is the only thing I corrected to make it work (I don't know how/why your code would compile):

pi.shipping_address = new ShippingAddress
{
   city = "San Mateo",
   country_code = "US",
   line1 = "SO TEST",
   line2 = "",
   postal_code = "94002",
   state = "CA",
 };

And finally the (successful) response from PayPal (ResponseJson):

ResponseJson: {
    "id": "PAY-8EU11336Y2032051SKUYZZOQ",
    "intent": "sale",
    "payer": {
        "payment_method": "credit_card",
        "funding_instruments": [{
            "credit_card": {
                "number": "xxxxxxxxxxxx2274",
                "type": "visa",
                "expire_month": 1,
                "expire_year": 2018,
                "first_name": "Stack",
                "last_name": "Overflow",
                "billing_address": {
                    "line1": "SO TEST",
                    "line2": "",
                    "city": "San Mateo",
                    "country_code": "US",
                    "postal_code": "94002",
                    "state": "CA"
                }
            }
        }]
    },
    "transactions": [{
        "related_resources": [{
            "sale": {
                "id": "1Y28050325502832C",
                "amount": {
                    "currency": "USD",
                    "total": "99.00"
                },
                "state": "completed",
                "parent_payment": "PAY-8EU11336Y2032051SKUYZZOQ",
                "create_time": "2015-04-17T23:52:26Z",
                "update_time": "2015-04-17T23:52:28Z",
                "links": [{
                    "href": "https://api.sandbox.paypal.com/v1/payments/sale/1Y28050325502832C",
                    "rel": "self",
                    "method": "GET"
                },
                {
                    "href": "https://api.sandbox.paypal.com/v1/payments/sale/1Y28050325502832C/refund",
                    "rel": "refund",
                    "method": "POST"
                },
                {
                    "href": "https://api.sandbox.paypal.com/v1/payments/payment/PAY-8EU11336Y2032051SKUYZZOQ",
                    "rel": "parent_payment",
                    "method": "GET"
                }]
            }
        }],
        "amount": {
            "currency": "USD",
            "total": "99.00",
            "details": {
                "subtotal": "99.00"
            }
        },
        "description": "trnx desc",
        "item_list": {
            "items": [{
                "quantity": "1",
                "name": "foo",
                "price": "99.00",
                "currency": "USD",
                "sku": "fooSku"
            }]
        }
    }],
    "state": "approved",
    "create_time": "2015-04-17T23:52:26Z",
    "update_time": "2015-04-17T23:52:28Z",
    "links": [{
        "href": "https://api.sandbox.paypal.com/v1/payments/payment/PAY-8EU11336Y2032051SKUYZZOQ",
        "rel": "self",
        "method": "GET"
    }]
}

And shown in Paypal developer Sandbox Transactions: