Oracle SCN_TO_TIMESTAMP (ORA_ROWSCN) fails
SCN_TO_TIMESTAMP


  The association between an SCN and a timestamp when the SCN is
  generated is remembered by the database for a limited period of time.
  This period is the maximum of the auto-tuned undo retention period, if
  the database runs in the Automatic Undo Management mode, and the
  retention times of all flashback archives in the database, but no less
  than 120 hours. The time for the association to become obsolete
  elapses only when the database is open. An error is returned if the
  SCN specified for the argument to SCN_TO_TIMESTAMP is too old.


Meaning the databases does not keep track of SCN-timestamp map forever and it is normal for SCN_TO_TIMESTAMP to fail for "too old" values.
With the SCN returned from ORA_ROWSCN, you can still try to find the period of the change from views such as V$LOG_HISTORY (FIRST_CHANGE#, NEXT_CHANGE#, FIRST_TIME, NEXT_TIME).

When the time of the last update is required, the actual value is typically maintained and stored in an extra column in the table. ORA_ROWSCN is not good enough for that.