I am getting CORS error on heroku in Node js

I have deployed my blog app on heroku, I started with 2 folders api and client after that I put my client folder in api folder and delpoyed on heroku.

I am getting an error from cors despite all efforts.

Can you let me know what am i doing wrong

Access to XMLHttpRequest at 'https://blogapi556.herokuapp.com/api/posts' from origin 'https://blogapp556.herokuapp.com' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource.

Server Js looks like this.

const express = require("express");
const app = express();
const dotenv = require("dotenv");
const mongoose = require("mongoose");
const authRoute = require("./routes/auth");
const userRoute = require("./routes/users");
const postRoute = require("./routes/posts");
const categoryRoute = require("./routes/categories");
const multer = require("multer");
const path = require("path");
let port = process.env.PORT || 5000;
dotenv.config();

app.use(function(req, res, next) {
  res.header("Access-Control-Allow-Origin", "*");
  next();
});
app.use(express.json());
app.use("/images", express.static(path.join(__dirname, "/images")));
  
mongoose
  .connect(process.env.MONGO_URI, {
    useNewUrlParser: true,
    useUnifiedTopology: true,
    useCreateIndex: true,
  })
  .then(console.log("Connected to MongoDB"))
  .catch((err) => console.log(err));
const storage = multer.diskStorage({
  destination: (req, file, cb) => {
    cb(null, "images");
  },
  filename: (req, file, cb) => {
    cb(null, req.body.name);
  },
});

const upload = multer({ storage: storage });
app.post("/api/upload", upload.single("file"), (req, res) => {
  res.status(200).json("File has been uploaded");
});
app.use("/api/auth", authRoute);
app.use("/api/users", userRoute);
app.use("/api/posts", postRoute);
app.use("/api/categories", categoryRoute);

app.use(express.static(path.join(__dirname, "/client/build")));

app.get('*', (req, res) => {
  res.sendFile(path.join(__dirname, '/client/build', 'index.html'));
});


app.listen(port, () => {
  console.log("Backend is running.");
});

The request in client looks like this

  const fetchPosts = async () =>{
    const res = await axiosInstance.get("/posts" + search);
    console.log(res);
setPosts(res.data)
  }

Axios Config File like this

import axios from "axios"

 const axiosInstance = axios.create({
    baseURL: "https://blogapi556.herokuapp.com/api/",
})

axiosInstance.defaults.headers.common['Access-Control-Allow-Origin'] = '*';


export {axiosInstance}