ALTER INDEX DBS.IDX_1              REBUILD;
Index altered.


ALTER INDEX DBS.IDX_4              REBUILD;
Index altered.

05:34:10 SQL> ALTER INDEX DBS.IDX_2              REBUILD;
ALTER INDEX DBS.IDX_2              REBUILD
*
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specified


Elapsed: 00:00:00.00
05:34:17 SQL>

select * from v$lock where request!=0;

no rows selected.

SELECT object_name, o.object_id FROM USER_OBJECTS o, v$locked_object l
 WHERE o.object_id = l.object_id

no rows selected.

SELECT ID1, ID2, TYPE FROM v$lock WHERE SID = (SELECT SID FROM v$mystat WHERE ROWNUM = 1);

no rows selected.

  SELECT s.SID, s.serial#, p.spid 
FROM 
   v$session s, 
   v$process p 
WHERE 
   s.paddr = p.addr 
AND 
   s.SID IN (SELECT SESSION_ID FROM v$locked_object);

Shows me 1 session

SELECT OBJECT_NAME,OBJECT_ID 
FROM DBA_OBJECTS 
WHERE OBJECT_NAME='DBS' 
AND owner='BASEL' 
AND OBJECT_TYPE='TABLE';

OBJECT_ID
---------
54831

SELECT SESSION_ID FROM v$locked_object WHERE OBJECT_ID='65831';

SESSION_ID
---------
78

SELECT * FROM v$SESSION WHERE SID IN (78)

Then i killed that session on unix command prompt.

kill -9 spid

Then i again tried to rebuild and it works.