How to disable the "read only mode" in hdfs web ui?
Safemode for the NameNode is essentially a read-only mode for the HDFS cluster
https://hadoop.apache.org/docs/r2.8.3/hadoop-project-dist/hadoop-hdfs/HdfsUserGuide.html#Safemode
You can try to leave it but without more information, your cluster is currently in a bad state, and you need to fix whatever the issue is (such as removing corrupt blocks) before it'll let you attempt to write to disk again.
How to implement simple authentification
Without Kerberos within Hadoop or some other proxy software in front of the Hadoop UI that adds Basic Auth, there is no password protected setup.