ORA-01940: cannot drop a user that is currently connected

When I was trying to drop a schema, I got below error.

SQL> drop user obi_dw cascade;

drop user obi_dw cascade

*

ERROR at line 1:

ORA-01940: cannot drop a user that is currently connected

Find out the session for connected user.

SQL> SELECT sid, serial#,status,username, logon_time,(last_call_et/60/60/24) "Days Inactive" FROM v$session WHERE username ='OBI_DW';

       SID    SERIAL# STATUS   USERNAME                       LOGON_TIM Days Inactive

---------- ---------- -------- ------------------------------ --------- -------------

        10         15 INACTIVE OBI_DW                         11-MAY-20    .000381944

Kill the session.

SQL> alter system kill session '10,15';

System altered.

Try to drop the user. 

SQL> drop user obi_dw cascade;

User dropped.

SQL>