Mongodb – MongooseError: The `uri` parameter to `openUri()` must be a string, got "undefined"

i’m new in mongo and i try do make a connection but i receive an error that i don’t kwon how to solve, the error is the below:

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. did not connect
(node:21807) [MONGOOSE] DeprecationWarning: Mongoose: the `strictQuery` option will be switched back to `false` by default in Mongoose 7. Use `mongoose.set('strictQuery', false);` if you want to prepare for this change. Or use `mongoose.set('strictQuery', true);` to suppress this warning.
(Use `node --trace-deprecation ...` to show where the warning was created)

my code is the follow:

index.js

    /* 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`));

.env file

MONGO_URL= 'mongodb+srv://User:[email protected]/?retryWrites=true&w=majority'
PORT = 3001

I think that MONGO_URL is not see like a string but i don’t understand why, I have already try to use :

require('dotenv').config();

but in receive this error :

ReferenceError: require is not defined in ES module scope, you can use import instead
This file is being treated as an ES module because it has a '.js' file extension and '/Users/gianlca/Desktop/progetto/server/package.json' contains "type": "module". To treat it as a CommonJS script, rename it to use the '.cjs' file extension.

i can’t rename it like ‘.cjs’ cause the import that i have in the file they start to not work too