Does undotbs_01 exist? Is it UNDO type? I think one of these question has an answer = NO :-)

If no UNDO tablespace exists, edit the init.ora file to comment out the UNDO_MANAGEMENT parameter: the default value for UNDO_MANAGEMENT is MANUAL.

Then, after starting the database, do one of the following:

A. Create a non UNDO tablespace with at least one online rollback segment. The instance can work this way.

-OR-

B. Create a new UNDO tablespace. If you want the instance to work under Automatic Undo Management,
a. Set the UNDO_MANAGEMENT back to AUTO
b. Set the UNDO_TABLESPACE parameter to the name of the new UNDO tablespace
c. Restart the database