braintree + NodeJS gives an "authentication Error " - node.js

Question

I'm trying to build a payment gateway library using braintree's NodeJS library, I'm making an ajax call from front end with the card data.
Client-Side
var card_data = {
type: "AmEx",
number: "3XXXXXXXXXXXXXX",
expire_month: "XX",
expire_year: "201X",
cvv2: "XXX",
name: "sandeep",
price : "200",
currency : "USD"
};
Ajax call,
$.ajax({
method: "GET",
url: "http://localhost:3000/paymentPath_braintree",
data: card_data
}).done(function(message){
console.log(message);
}).fail(function(data, message){
console.log(message);
});
Server-Side
var braintreePay = require('braintree');
app.get("/payment_braintree", function(request, response){
var data = request.query;
var gateway = braintreePay.connect({
environment: braintreePay.Environment.Sandbox,
merchantId: "MymerchentID",
publicKey: "MypublicKey",
privateKey: "MyprivateKey",
});
var saleRequest = {
amount: data.price,
creditCard: {
number: data.number,
cvv: data.cvv2,
expirationMonth: data.expire_month,
expirationYear: data.expire_year.slice(2),
cardHolder: data.name
},
options: {
submitForSettlement: true
}
};
gateway.transaction.sale(saleRequest, function(error, result){
if(error){
console.log(error.name);
throw error;
}
if(result.success){
response.send(result.transaction.id);
} else {
response.send(result.message);
}
});
});
I have cross checked everything, from keys and card data everything is in order but i am getting an error in callback after making gateway.transaction.sale(...); called Authentication Error . Which i tried to figure out for hours but could not get through.
error object is
authenticationError: Authentication Error
arguments: undefined
message: "Authentication Error"
name: "authenticationError"
stack: undefined
type: "authenticationError"
Where am i going wrong?
I have created an account sandbox.braintreegateway those key credentials are from the account that i have created, i din't create an app like how its done in paypal.
I am going through lack of understanding in Braintree integration.
Are their any proper documented resource.