"Senior and pensioner tax offset" can be found via "Estimate" tab.
Make sure birthdate is correct, or adjust it via "Settings>General"
If you need to update the birthdate, make sure that the form is not lock/completed. (you can find this at top right of the form, click the lock icon)
Note: In order to claim senior tax offset of the spouse, use the option "Eligible spouse lived together" via "Offset" tab