title: Cannot login as an admin user in Mongo 3.6
body: <p>I'm simply trying to create an admin user in mongo 3.6 and log in. I ran:</p>
  
  <pre><code> $mongo
  db.createUser(
   {
     user:"AdminMark",
     pwd:"fghtyyy",
     roles:[{role:"root",db:"admin"}]
   }
  )
  </code></pre>
  
  <p>It says: <code>Successfully added user</code></p>
  
  <p>Then tried to log in:</p>
  
  <pre><code>mongo -u AdminMark -p --authenticationDatabase admin
  </code></pre>
  
  <p>And I enter password. But I get:</p>
  
  <pre><code> 2018-03-05T20:40:07.689+0000 E QUERY    Error: 18 Authentication failed.
  at DB._authOrThrow (src/mongo/shell/db.js:1271:32)
  at (auth):6:8
  at (auth):7:2 at src/mongo/shell/db.js:1271
  exception: login failed
  </code></pre>
  
  <p>This happens but locally and on my server. What's going on?</p>
  
  <p>Im following the tutorial here: <a href="https://tecadmin.net/create-drop-users-in-mongodb/#" rel="nofollow noreferrer">https://tecadmin.net/create-drop-users-in-mongodb/#</a></p>
  
  <p>The contents of my mongod.conf file is:</p>
  
  <pre><code>    # mongod.conf
  
      # for documentation of all options, see:
      #   http://docs.mongodb.org/manual/reference/configuration-options/
  
      # Where and how to store data.
      storage:
        dbPath: /var/lib/mongodb
        journal:
          enabled: true
      #  engine:
      #  mmapv1:
      #  wiredTiger:
  
      # where to write logging data.
      systemLog:
        destination: file
        logAppend: true
        path: /var/log/mongodb/mongod.log
  
      # network interfaces
      net:
        port: 27017
        bindIp: 127.0.0.1
  
  
      #processManagement:
  
      #security:
  
      #operationProfiling:
  
      #replication:
  
      #sharding:
  
      ## Enterprise-Only Options:
  
      #auditLog:
  
      #snmp:
  </code></pre>
  
  <p>I also tried <a href="https://stackoverflow.com/a/33372968/1246159">this</a>. Didn't work. </p>
  
  <p>I can I very simply add a mongo 3.6 admin user, then log in to Mongo?</p>