I can't connect to MongoDB using mongoose - node.js

Question

I keep getting a error "MongooseError: The uri parameter to openUri() must be a string, got "undefined". Make sure the first parameter to mongoose.connect() or mongoose.createConnection() is a string.". I've been looking at other questions that get a similar error butI did not find a solution after trying multiple different things.
in package.json
{
"dependencies": {
"bcrypt": "^5.1.0",
"body-parser": "^1.20.1",
"cors": "^2.8.5",
"dotenv": "^16.0.3",
"express": "^4.18.2",
"gridfs-stream": "^1.1.1",
"helmet": "^6.0.1",
"jsonwebtoken": "^9.0.0",
"mongoose": "^6.7.0",
"morgan": "^1.10.0",
"multer": "^1.4.4",
"multer-gridfs-storage": "^5.0.2"
},
"name": "server",
"version": "1.0.0",
"main": "index.js",
"type": "module",
"devDependencies": {},
"scripts": {
"test": "echo \"Error: no test specified\" && exit 1"
},
"keywords": [],
"author": "",
"license": "ISC",
"description": ""
}
in index.js
import express from "express";
import bodyParser from "body-parser";
import mongoose from "mongoose";
import cors from "cors";
import dotenv from "dotenv";
import multer from "multer";
import helmet from "helmet";
import morgan from "morgan";
import path from "path";
import { fileURLToPath } from "url";
/* CONFIGURATIONS */
const __filename = fileURLToPath(import.meta.url);
const __dirname = path.dirname(__filename);
dotenv.config();
const app = express();
app.use(express.json());
app.use(helmet());
app.use(helmet.crossOriginResourcePolicy({ policy: "cross-origin" }));
app.use(morgan("common"));
app.use(bodyParser.json({ limit: "30mb", extended: true }));
app.use(bodyParser.urlencoded({ limit: "30mb", extended: true }));
app.use(cors());
app.use("/assets", express.static(path.join(__dirname, "public/assets")));
/* FILE STORAGE */
const storage = multer.diskStorage({
destination: function (req, file, cb) {
cb(null, "public/assets");
},
filename: function (req, file, cb) {
cb(null, file.originalname);
},
});
const upload = multer({ storage });
/* MONGOOSE SETUP */
const PORT = process.env.PORT || 6001;
mongoose
.connect(process.env.MONGO_URL, {
useNewUrlParser: true,
useUnifiedTopology: true,
})
.then(() => {
app.listen(PORT, () => console.log(`Server Port: ${PORT}`));
})
.catch((error) => console.log(`${error} did not connect`));
in .env
MONGO_URL = 'the connection string'
PORT = 3001