Error While Login Access token not working

General
1 
import React, { useState, useEffect } from "react";

const App = () => {
  const [authCode, setAuthCode] = useState("");

  const handleAuth = () => {
    const clientId = "";
    const redirectUri = "http://127.0.0.1:3000";
    const authUrl = `https://api.upstox.com/v2/login/authorization/dialog?response_type=code&client_id=${clientId}&redirect_uri=${redirectUri}`;
    window.location.href = authUrl;
  };

  useEffect(() => {
    const params = new URLSearchParams(window.location.search);
    const code = params.get("code");

    if (code) {
      setAuthCode(code);
    }
  }, []);

  useEffect(() => {
    if (authCode) {
      fetch("http://127.0.0.1:3001/api/token", {
        method: "POST",
        headers: { "Content-Type": "application/x-www-form-urlencoded" },
        body: `code=${authCode}&grant_type=authorization_code`,
      })
        .then((response) => response.json())
        .then((data) => {
          console.log(data);
          const token = data.access_token;
          console.log("Extracted access token:", token);
          // Save token to localStorage
          localStorage.setItem("accessToken", token);
        })
        .catch((error) => console.error("error at access-token", error));
    }
  }, [authCode]);

  return (
    <div className="container-fluid m-0 p-0">
      <h1 className="text-success fs-1 text-center mt-1">
        Invest in knowledge before investing in stocks{" "}
        <span>
          <p
            className="text-end pe-4 "
            style={{
              fontSize: "9px",
              fontWeight: "200",
            }}
          >
          Data
          </p>
        </span>
      </h1>
      <div className="d-flex justify-content-center pt-4">
        <button className="btn btn-outline-info" onClick={handleAuth}>
          Authenticate
        </button>
      </div>
    </div>
  );
};

export default App;

above is frontend code

below is backend code

const express = require("express");
const axios = require("axios");
const dotenv = require("dotenv");
const cors = require("cors");
const bodyParser = require("body-parser");

dotenv.config();

const app = express();
app.use(cors());
app.use(bodyParser.urlencoded({ extended: true }));
app.use(bodyParser.json());

const port = 3001;
let accessToken = null; 

// Route to handle login and store the access token
app.post("/api/token", async (req, res) => {
  const { code } = req.body;

  try {
    const response = await axios.post(
      "https://api.upstox.com/v2/login/authorization/token",
      null,
      {
        params: {
          code,
          client_id: process.env.CLIENT_ID,
          client_secret: process.env.CLIENT_SECRET,
          redirect_uri: process.env.REDIRECT_URI,
          grant_type: "authorization_code",
        },
        headers: {
          "Content-Type": "application/x-www-form-urlencoded",
          Accept: "application/json",
        },
      }
    );

    accessToken = response.data.access_token; // Store access token
    console.log("Access token stored:", accessToken);
    res.json({ access_token: accessToken });
  } catch (error) {
    console.error(
      "Error getting access token:",
      error.response ? error.response.data : error.message
    );
    res.status(500).send("Error getting access token");
  }
});

// Start server
app.listen(port, () => {   console.log(`Server running at http://127.0.0.1:${port}`);
});

why i am not able to generate access token ??