you dont have permission to access https://securegw-stage.paytm.in/theia/api/v1/showPaymentPage? on this server

Question

I am trying to integrate paytm payment gateway in my flutter application by following all in one sdk developer documentation but getting the error on UI (Access Denied you don't have permission to access https://securegw-stage.paytm.in/theia/api/v1/showPaymentPage?mid=MID&orderId=ORDER_ID on this server)

Here is my payment config class code

import 'dart:convert';

import 'package:flutter/services.dart';
import 'package:http/http.dart' as http;
import 'package:paytm_allinonesdk/paytm_allinonesdk.dart';

class PaytmConfig {
  final String _mid = MID;
  final String _mKey = MKEY;
  final String _website = "WEBSTAGING";
  final String _url =
      'http://flutter-paytm-backend.great-site.net/generateTxnToken'; //testing server

  String get mid => _mid;
  String get mKey => _mKey;
  String get website => _website;
  String get url => _url;

  String getMap(double amount, String callbackUrl, String orderId) {
    return json.encode({
      "mid": mid,
      "key_secret": mKey,
      "website": website,
      "orderId": orderId,
      "amount": amount.toString(),
      "callbackUrl": callbackUrl,
      "custId": "9204473819624814",
    });
  }

  Future<void> generateTxnToken(double amount, String orderId) async {
    final callBackUrl =
        'https://securegw-stage.paytm.in/theia/paytmCallback?ORDER_ID=$orderId';
    final body = getMap(amount, callBackUrl, orderId);

    try {
      final response = await http.post(
        Uri.parse(url),
        body: body,
        headers: {'Content-type': "application/json"},
      );
      String txnToken = response.body;

      await initiateTransaction(orderId, amount, txnToken, callBackUrl);
    } catch (e) {
      print(e);
    }
  }

  Future<void> initiateTransaction(String orderId, double amount,
      String txnToken, String callBackUrl) async {
    String result = '';
    try {
      var response = AllInOneSdk.startTransaction(
        mid,
        orderId,
        amount.toString(),
        txnToken,
        callBackUrl,
        true,
        false,
      );
      response.then((value) {
        // Transaction successfull
        print(value);
      }).catchError((onError) {
        if (onError is PlatformException) {
          result = onError.message! + " \n  " + onError.details.toString();
          print(result);
        } else {
          result = onError.toString();
          print(result);
        }
      });
    } catch (err) {
      // Transaction failed
      result = err.toString();
      print(result);
    }
  }
}

Calling the function to initiate the transaction when the button is pressed

ElevatedButton(
                  onPressed: () async => {
                    if (_formKey.currentState!.validate())
                      {
                        await PaytmConfig().generateTxnToken(
                            double.parse(priceController.text.toString()),
                            ORDER_ID),
                      }
                  },
                  child: Text('Pay'),
                )

And here is my app.js class code in backend

var express = require('express');
var port = process.env.PORT || 3000;
var app = express();

const https = require('https');

const checksum_lib = require('./checksum');


app.use(express.json()); 
app.use(express.urlencoded());

app.get('/', function(req, res) {
    console.log(req);
    res.send(JSON.stringify({ Hello: 'World' }));
});
app.post('/generateTxnToken', function(request, res) {
    console.log(request.body);

    var paytmParams = {};

    var MID = request.body.mid;
    var orderId = request.body.orderId;

    var amount = parseFloat(String(request.body.amount));
    var custId = request.body.custId;
    var key_secret = request.body.key_secret;
    var callbackUrl = request.body.callbackUrl;
    var mode = request.body.mode;
    var website = request.body.website;
    var testing = String(request.body.testing);
    console.log(callbackUrl);
    console.log(mode);


    paytmParams.body = {

        /* for custom checkout value is 'Payment' and for intelligent router is 'UNI_PAY' */
        "requestType": "Payment",

        /* Find your MID in your Paytm Dashboard at https://dashboard.paytm.com/next/apikeys */
        "mid": MID,

        /* Find your Website Name in your Paytm Dashboard at https://dashboard.paytm.com/next/apikeys */
        "websiteName": website == undefined ? "DEFAULT" : website,

        /* Enter your unique order id */
        "orderId": orderId,

        /* on completion of transaction, we will send you the response on this URL */
        // "callbackUrl": "https://mrdishant.com",
        "callbackUrl": callbackUrl,

        /* Order Transaction Amount here */
        "txnAmount": {

            /* Transaction Amount Value */
            "value": amount,

            /* Transaction Amount Currency */
            "currency": "INR",
        },

        /* Customer Infomation here */
        "userInfo": {

            /* unique id that belongs to your customer */
            "custId": custId,
        },

    };

    console.log("Mode");
    console.log(mode);

    if (mode == "1") {
        console.log("Mode 1 So Net Banking");
        paytmParams.body[
            "enablePaymentMode"] = [{
            "mode": "NET_BANKING",
        }]
    } else if (mode == "0") {
        console.log("Mode 0 So BALANCE");
        paytmParams.body[
            "enablePaymentMode"] = [{
            "mode": "BALANCE",
        }]
    } else if (mode == "2") {
        console.log("Mode 2 So UPI");
        paytmParams.body[
            "enablePaymentMode"] = [{
            "mode": "UPI",
        }]
    } else if (mode == "3") {
        console.log("Mode 3 So CC");
        paytmParams.body[
            "enablePaymentMode"] = [{
            "mode": "CREDIT_CARD"
        }]
    }

    console.log(JSON.stringify(paytmParams));

    /**
     * Generate checksum by parameters we have in body
     * Find your Merchant Key in your Paytm Dashboard at https://dashboard.paytm.com/next/apikeys 
     */
    checksum_lib.genchecksumbystring(JSON.stringify(paytmParams.body), key_secret, (err, checksum) => {

        if (err) {
            return;
        }

        /* head parameters */
        paytmParams.head = {

            /* put generated checksum value here */
            "signature": checksum
        };

        /* prepare JSON string for request */
        var post_data = JSON.stringify(paytmParams);

        var options = {



            /* for Staging */


            /* for Production */
            hostname: testing == "0" ? 'securegw-stage.paytm.in' : 'securegw.paytm.in',

            port: 443,
            path: '/theia/api/v1/initiateTransaction?mid=' + MID + '&orderId=' + orderId,
            method: 'POST',
            headers: {
                'Content-Type': 'application/json',
                'Content-Length': post_data.length
            }
        };

        // Set up the request
        var response = "";
        var post_req = https.request(options, (post_res) => {
            post_res.on('data', (chunk) => {
                response += chunk;
            });

            post_res.on('end', () => {
                console.log(orderId);
                console.log(MID);
                console.log('Response: ', response);
                response = JSON.parse(response);
                res.send(response.body.txnToken);
                return 0;
            });
        });

        // post the data
        post_req.write(post_data);
        post_req.end();
    });
});

app.listen(port, function() {
    console.log(`Example app listening on port !`);
});