AUTOEXTENSIBLE is a feature at datafile level (not tablespace level)

If your SYSTEM tablespace (meaning the datafiles underneath) is almost full you can do any of the following --

1. RESIZE the datafile to increase in size --

ALTER DATABASE DATAFILE 'C:\ORACLE\ORADATA\DBA1\SYSTEM01.DBF' RESIZE 200000K

2. Add a new datafile to the System tablespace --

ALTER TABLESPACE "SYSTEM" ADD DATAFILE 'C:\ORACLE\ORADATA\DBA1\SYSTEM02.DBF' SIZE 10M

3. Set the AUTOEXTEND on --

ALTER DATABASE DATAFILE 'C:\ORACLE\ORADATA\DBA1\SYSTEM01.DBF' AUTOEXTEND ON NEXT 20K

- Rajeev