ORA-00027 cannot kill current session .

ORA-00027 cannot kill current session
Cause: An attempt was made to use ALTER SYSTEM KILL SESSION to kill the
current session.
Action: If it is necessary to kill the current session, do so from another session.

ORA-00027 comes under "Oracle Database Server Messages". 

These messages are generated by the Oracle database server when running any Oracle program.

How to kill session:
1. identify which session to kill using following query:
 select 
  s.sid,
  s.serial#,
  spid,
  trim(s.machine) machine,
  trim(s.module) module,
  status
 from
  v$session s,
  v$process p
 where
  paddr=addr
  and module is not null
 order by 1,2
 
2. Killing a session
 ALTER SYSTEM KILL SESSION 'sid,serial#';
 
 or you can kill at OS level on linux using
 kill -9 spid