APP.js in backened

import express from "express";
import userRouter from "./routes/user.js";
import taskRouter from "./routes/task.js";
import { config } from "dotenv";
import cookieParser from "cookie-parser";
import { errorMiddleware } from "./middlewares/error.js";
import cors from "cors";

export const app = express();

config({
  path: "./data/config.env",
});

// Using Middlewares
app.use(express.json());
app.use(cookieParser());

app.use(
  cors({
    // origin: ["http://127.0.0.1:5174/api/v1"],
    origin: ["http://localhost:3000/api/v1"],
    methods: ["GET", "POST", "PUT", "DELETE"],
    credentials: true,
  })
);

// Using routes
app.use("/api/v1/users", userRouter);
app.use("/api/v1/task", taskRouter);

app.get("/", (req, res) => {
  res.send("Nice working");
});

// Using Error Middleware
app.use(errorMiddleware);

main.jsx from frontened

import React, { useState } from "react";
import ReactDOM from "react-dom/client";
import App from "./App";
import "./styles/app.scss";
import { createContext } from "react";

export const server = "https://mern-todoapp-bc0e.onrender.com/api/v1";
// export const server = "https://localhost:3000/api/v1";

export const Context = createContext({ isAuthenticated: false });

const AppWrapper = () => {
  const [isAuthenticated, setIsAuthenticated] = useState(false);
  const [loading, setLoading] = useState(false);
  const [user, setUser] = useState({});


  return (
    <Context.Provider
      value={{
        isAuthenticated,
        setIsAuthenticated,
        loading,
        setLoading,
        user,
        setUser,
      }}
    >
      <App />
    </Context.Provider>
  );
};

ReactDOM.createRoot(document.getElementById("root")).render(
  <React.StrictMode>
    <AppWrapper />
  </React.StrictMode>
);

在'https：//mern-todoapp-bc 0 e访问XMLHttpRequest。onrender.com/api/v1/users/new' from origin ' http：//127。“0.0.1：5174”已被CORS策略阻止：对印前检查请求的响应未通过访问控制检查：请求的资源上不存在“Access-Control-Allow-Origin”标头。
为什么我得到这个错误，而登录和登录在我的网页，甚至我已经使用了所有的cors完美？？
我已经在这个链接上部署了我的API“ www.example.com ”，并且其工作正常，但无法解决cors策略错误