$ORACLE_HOME/bin/sqlplus -S /NOLOG
After lot of Investigation we found that the problem is setting the env. variables for 11gR2 ORACLE_HOME.
Do not set any env variable related to both oracle homes 10g nor 11g and execute dbua manually from 11gR2 home path
The env where dbua worked for me:
Hope this should solve the issue.
Thanks for reading :)