To run the database in automatic undo management mode, you must set undo_management to auto from the oracle startup parameters.
You can use the following script to change it.
ALTER SYSTEM SET UNDO_MANAGEMENT=AUTO SCOPE=SPFILE;
For a database running on multiple instances in the Rack structure, you must change it as follows.
ALTER SYSTEM SET UNDO_MANAGEMENT=AUTO SID='*' SCOPE=SPFILE;