React Nodejs Express Server Cannot GET - node.js

Question

I'm developing my first full-stack project. I developed the client side and server side. You can see the code down. When I run the client (Front end side ) it is working in localhost. Then when I ran the backend side I got a 'Cannot GET' message. Any idea about this?
Index.js
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");
dotenv.config();
app.use(express.json());
app.use("/images", express.static(path.join(__dirname, "/images")));
mongoose.connect(process.env.MONGO_URL).then(console.log('Connected to mongo')).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.listen("3000", () => {
console.log("Backend is running.");
});
Home.jsx
import React from 'react'
import Header from '../../components/header/Header'
import Posts from '../../components/posts/Posts'
import Sidebar from '../../components/sidebar/Sidebar'
import axios from "axios";
import { useEffect, useState } from "react";
export default function Home() {
const [posts, setPosts] = useState([]);
useEffect(() => {
const fetchPosts = async () => {
const res = await axios.get("/posts");
console.log(res)
setPosts(res.data);
};
fetchPosts();
});
return (
<>
<div className="mt-8">
<Header/>
</div>
<div className="container">
<div className="row mt-5">
<div className="col-lg-4 col-md-4">
<Posts posts={posts}/>
</div>
<div className="col-lg-4 col-md-4 ms-auto">
<Sidebar/>
</div>
</div>
</div>
</>
)
}
I added proxy to package.json file. (Client side).
"proxy": "http://localhost:3000/api/",