server returned error on SASL authentication step: Authentication failed

Before this, make sure you’re logged in to heroku CLI. If not, you may login by running heroku login. Now use the heroku config command to view your app’s config variables.The output of the command would be something like this:

mongodb://aaaaaa:bbbbbb@ds029017.mLab.com:29017/heroku_12345678

Now to get user information from this URI, map this string to the following one:

mongodb://dbuser:dbpass@host:port/dbname

This way, you can get your information from URI.
Thanks for your time.