You might get this error, when you tried to drop the undo table. Its because , there are some active transactions
in the undo. The solution is to find that transaction and kill the same.

set pagesize 200
set lines 200
set long 999
col username for a9
SELECT,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
  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.