It is because the parent directories do not exist yet either. Try hdfs dfs -mkdir -p /user/Hadoop/twitter_data. The -p flag indicates that all nonexistent directories leading up to the given directory are to be created as well.
As for the question you posed in the comments, simply type into your browser http://<host name of the namenode>:<port number>/
And also I suggest you to try this as well
use the below steps command to create the directory:
1) don't run the hadoop and format the namenode:-
$ hadoop namenode -format
2) run hadoop by :-
3)now first make the initial directory then create the another in same directory:
$ hadoop fs -mkdir /user
$ hadoop fs -mkdir /user/Hadoop
$ hadoop fs -mkdir /user/Hadoop/tweeter_data
Follow the above steps to solve the problem.