ORA-00027: cannot kill current session , error comes, when you are trying to kill your current session,which is not allowed. 

EXAMPLE:

Get the sid and serial# for the current session.

SQL> select username,inst_id, sid, serial# FROM gV$SESSION WHERE audsid = userenv(‘sessionid’);

USERNAME INST_ID SID SERIAL#
—————————— ———- ———- ———-
SYSTEM 1 242 13

Try to kill the session from current session:

SQL> alter system kill session ‘242,13’ immediate;
alter system kill session ‘242,13’ immediate
*
ERROR at line 1:
ORA-00027: cannot kill current session

Now  open another fresh session and run the ALTER SYSTEM KILL statement, it will execute successfully.

SQL> alter system kill session ‘242,13’ immediate; 

Session Killed