There is not enough space left either due to the datafiles being full, autoextend which is not set at datafile level or due to a disk which's full.

You'll have to check the size of the datafiles attached to the undo tablespace and check whether they can autoextend or not..[@more@]

SQL> delete from data where sid=41;
delete from data where sid=41
*
ERROR at line 1:
ORA-30036: unable to extend segment by 128 in undo tablespace 'UNDOTBS1'

SQL> select file_name, bytes, autoextensible, maxbytes from dba_data_files where tablespace_name='UNDOTBS1';

FILE_NAME
--------------------------------------------------------------------------------
BYTES AUTOEX MAXBYTES
---------- ------ ----------
/home/oracle/oradata/depsight/undotbs1.dbf

SQL> alter database datafile '/home/oracle/oradata/depsight/undotbs1.dbf' resize 50m;

Database altered.

SQL> select file_name, bytes, autoextensible, maxbytes from dba_data_files where tablespace_name='UNDOTBS1';

FILE_NAME
--------------------------------------------------------------------------------
BYTES AUTOEX MAXBYTES
---------- ------ ----------
/home/oracle/oradata/depsight/undotbs1.dbf
52428800 NO 0


SQL> delete from data where sid=41;
delete from data where sid=41