Mongo Auth fail when using mongod.conf
As per your log statement it seems like that you are committing mistake in your authentication statement such as in your statement

mongo 127.0.0.1:27017 --authenticationDatabase admin -u admin -p


As per MongoDB BOL documentation here With access control enabled, ensure you have a user with userAdmin or userAdminAnyDatabase role in the admin database. This user can administrate user and roles such as: create users, grant or revoke roles from users, and create or modify customs roles.

Connect and authenticate as the user administrator

Using the mongo shell, you can:


Connect with authentication by passing in user credentials, or
Connect first withouth authentication, and then issue the  db.auth()
method to authenticate.


For example, the following creates the user myUserAdmin in the admin database:

use admin
db.createUser(
  {
    user: "myUserAdmin",
    pwd: "abc123",
    roles: [ { role: "userAdminAnyDatabase", db: "admin" } ]
  }
)


To authenticate during connection

Start a mongo shell with the -u , -p , and the --authenticationDatabase  command line options:

mongo --port 27017 -u "myUserAdmin" -p "abc123" --authenticationDatabase "admin"


To authenticate after connecting

Connect the mongo shell to the  mongod:

mongo --port 27017


Switch to the authentication database (in this case, admin), and use db.auth(&lt;username&gt;, &lt;pwd&gt;) method to authenticate:

use admin
db.auth("myUserAdmin", "abc123" )


As I am able to see that in your below mention mongod.conf file the authorization : enable . It's means your authentication is enable. So, there is no issue in your mongod.conf file.

systemLog:
  destination: file
  path: /Users/me/Web/blog/setup/logs/mongodb/mongo.log
  logAppend: true
storage:
  dbPath: /usr/local/var/mongodb
net:
  port: 27017
  bindIp: 127.0.0.1
security:
  authorization: enabled


For your further ref here and here