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 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.