Make sure that your username and password are correct in the URI without any “<” or “>” symbols around them.
Make sure you are calling the environment variable correctly: process.env.MONGO_URI
If none of those things are causing the issue, using a different Node version usually fixes this error. When you create the connection string, choose version: 2.2.12 or later. Then paste the new string in the .env file and ensure steps 1-3 are right.
Hopefully one of these steps clears it up for you!