The "authentication failed" error message is often the result of one of five conditions.
Password missing after copying connection string from the Atlas web page. This is a security precaution, please enter the password manually in the Compass connection window.
Username and password mismatch
No user account exists in a new Atlas project under Clusters: Security: MongoDB Users
Not specifying any or the correct "Authentication Database", (aka authSource), usually "admin" for Atlas clusters
Selecting an incorrect "Authentication" method
原文：https://www.cnblogs.com/zergling9999/p/14769016.html