Is ‘Internal S’ valid JSON?

When I post a request in a local host this code runs fine but in production, this shows me an error – VM49:1 Uncaught (in promise) SyntaxError: Unexpected token ‘I’, "Internal S"… is not valid JSON

Hosting :

Frontend – Netlify and Backend -Render

This is my frontend code:

const handleSubmit = async (e) => {
    e.preventDefault();
    const response = await fetch(`${BASE_URL}/api/auth/createuser`, {
      method: "POST",
      headers: {
        "Content-Type": "application/json",
      },
      body: JSON.stringify({
        "name": credentials.name,
        "email": credentials.email,
        "password": credentials.password,
      }),
    });

    const json = await response.json();
    console.log(json);
    if (json.success) {
      // Save the auth token and redirect
      navigate("/");
      props.showAlert("Account Created Successfully", "success");
    } else {
      props.showAlert("Invalid Details", "danger");
    }
  };

and my backend code is

const express = require("express");
const router = express.Router();
const User = require("../models/User");
const bcrypt = require("bcryptjs");
const jwt = require("jsonwebtoken");
const { body, validationResult } = require("express-validator");
const fetchuser = require('../middleware/fetchUser');

const JWT_SECRET = "marufisagoodb$oy";

// ROUTE 1: Create a User using: POST "/api/auth/signup". No login required
router.post(
  "/createuser",
  [
    body("name", "Enter a valid name").isLength({ min: 3 }),
    body("email", "Enter a valid email").isEmail(),
    body("password", "Password must be atleast 5 characters").isLength({
      min: 5,
    }),
  ],
  async (req, res) => {
    let success = false;
    // If there are errors, return Bad request and the errors
    const errors = validationResult(req);
    if (!errors.isEmpty()) {
      return res.status(400).json({success, errors: errors.array() });
    }
    // Check whether the user with this email exists already
    try {
      let user = await User.findOne({ email: req.body.email });
      if (user) {
        return res
          .status(400)
          .json({
            success,
            error: "Sorry a user with this email already exists",
          });
      }

      //Hashing a password using bcryptjs
      const salt = await bcrypt.genSalt(10);
      const securepass = await bcrypt.hash(req.body.password, salt);

      // Create a new user
      user = await User.create({
        name: req.body.name,
        password: securepass,
        email: req.body.email,
      });

      //JWT
      const data = {
        user: {
          id: user.id,
        },
      };
      const authtoken = jwt.sign(data, JWT_SECRET);
      // res.json(user)
      success = true
      res.json({ success, authtoken });
    } catch (error) {
      console.error(error.message);
      res.status(500).send("Internal Server Error");
    }
  }
);

// ROUTE 2: Authenticate a User using: POST "/api/auth/login". No login required
router.post(
  "/login",
  [
    body("email", "Enter a valid email").isEmail(),
    body("password", "Password cannot be blank").exists(),
  ],
  async (req, res) => {
    let success = false;
    // If there are errors, return Bad request and the errors
    const errors = validationResult(req);
    if (!errors.isEmpty()) {
      return res.status(400).json({ errors: errors.array() });
    }

    const { email, password } = req.body;
    try {
      let user = await User.findOne({ email });
      if (!user) {
        success = false;
        return res
          .status(400)
          .json({ error: "Please try to login with correct credentials" });
      }

      const passwordCompare = await bcrypt.compare(password, user.password);
      if (!passwordCompare) {
        success = false;
        return res.status(400).json({
          success,
          error: "Please try to login with correct credentials",
        });
      }

      const data = {
        user: {
          id: user.id,
        },
      };
      const authtoken = jwt.sign(data, JWT_SECRET);
      success = true;
      res.json({ success, authtoken });
    } catch (error) {
      console.error(error.message);
      res.status(500).send("Internal Server Error");
    }
  }
);

// ROUTE 3: Get loggedin User Details using: POST "/api/auth/getuser". Login required
router.post("/getuser", fetchuser, async (req, res) => {
  try {
    userId = req.user.id;
    const user = await User.findById(userId).select("-password");
    res.send(user);
  } catch (error) {
    console.error(error.message);
    res.status(500).send("Internal Server Error");
  }
});

module.exports = router;

Please solve this error