ORA-30013: Undo Tablespace ‘UNDOTBS1’ Is Currently In Use

 set pagesize 200
set lines 200
set long 999
col username for a9
SELECT a.name,b.status , d.username , d.sid , d.serial#
FROM v$rollname a,v$rollstat b, v$transaction c , v$session d
WHERE a.usn = b.usn
AND a.usn = c.xidusn
AND c.ses_addr = d.saddr
AND a.name IN (
SELECT segment_name
FROM dba_segments
WHERE tablespace_name = 'UNDOTBS1'
);
NAME STATUS USERNAME SID SERIAL#
---------- --------------- ----------- ---------- ----------
_SYSSMU691$ PENDING OFFLINE SCOTT 20 30

Now kill this sessions:

alter system kill session '20,30' immediate;

system altered

Now try to drop the undo tablespace.

SQL> drop tablespace undotbs1 including contents and datafiles;
Tablespace dropped.