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.

Identify which session to kill using following query :-
SQL> 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

Kill that session :-
ALTER SYSTEM KILL SESSION 'sid,serial#';

Or Kill at OS level on linux using
kill -9 spid