The "authentication failed" error message is often the result of one of five conditions.
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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.
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Username and password mismatch
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No user account exists in a new Atlas project under Clusters: Security: MongoDB Users
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Not specifying any or the correct "Authentication Database", (aka authSource), usually "admin" for Atlas clusters
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Selecting an incorrect "Authentication" method