You have a choice. You can use manual or automatic undo management. Manual uses rollback segments like the old days. Auto uses an undo tablespace with no rollback segments defined in it. My guess is you are trying to do something in between, hence you problem.

You can't define rollback segments in your init.ora is you are using automatic. you'll probably only have something like this:

UNDO_MANAGEMENT = AUTO # Default is MANUAL
UNDO_TABLESPACE = undotbs_01 # The name of the undo tablespace.
UNDO_RETENTION = 900 # The time undo is retained.
# Default is 900 seconds.
UNDO_SUPPRESS_ERRORS = TRUE # Suppress errors when MANUAL undo admin
# SQL statements are issued.

Take a look at this article and try again:

http://www.oracle-base.com/Articles/...Management.asp

Cheers