Is There A Way To Fix This Cors Error On Heroku

Question

I have a full stack app but when I uploaded my backend to heroku, on browsers cors policy is blocking my API routes. I get the error

No ‘Access-Control-Allow-Origin’ header is present on the requested resource.)

on heroku I get a 503 error for the routes. I am requesting https from an https origin on netlify which is my frontend. I tried to fix it but it does not solve all my cors errors.

index.js

const express = require("express");

const app = express();
const PORT = process.env.PORT || 8500
const mongoose = require("mongoose");
const dotenv = require("dotenv");
const helmet = require("helmet");
const morgan = require("morgan");
const bodyParser = require("body-parser");
const cookieParser = require("cookie-parser");
const errorHandler = require("./middleware/error");
const cors = require("cors");

// setting cors
const corsOptions = {
    origin: ['*', '*'],
    preflightContinue:false,
    credentials: true,
    methods: 'GET,HEAD,PUT,PATCH,POST,DELETE',
  }



//Import Routes
const Routes = require('./routes/routes');

dotenv.config();



//middleware
app.use(cors(corsOptions));
app.use(express.json());
app.use(helmet());
app.use(bodyParser.json());
app.use(morgan("dev"));
app.use(cookieParser());
app.use("/api", Routes);


//welcome message 
app.get('/', (req,res) => {
    console.log("Welcome to the API.")
});



app.get('/greeting', (req,res) => {
    res.json({greeting: 'Hello there'})
});


mongoose.connect(process.env.MONGO_URL, {useNewUrlParser: true, useUnifiedTopology: true}).then(() => {
    console.log("MongoDB Connected")
})
.catch((err) => console.log(err));



app.listen(PORT, ()=> {
    console.log("Backend server is running!")
})