https://docs.mongodb.com/manual/tutorial/authenticate-nativeldap-activedirectory/

Step 11 appears twice

In (the first) step 11, "Connect to the MongoDB server." The example should read:

mongo --username sam@DBA.EXAMPLE.COM --password secret123 --authenticationMechanism PLAIN --authenticationDatabase $external --host <hostname> --port <port>

// authenticationMechanism not authenticationMechanisms