Hello JOO
The Saver Day Pass alone does not qualify for a Day Class Upgrade. You would need the (Swiss) Half Fare Card in addition. With the Saver Day Pass (without Half Fare Card) you can only buy
- Point-to-Point class upgrades or
- Saver Point-to-point class upgrades.
Please see the conditions here: Class upgrade for a route or for a day | SBB
If you select “No discount” as reduction type during purchase, the error occurs as the Day Class Upgrade cannot be purchased without a discount, which would be (Swiss) Half Fare Card.
Kind regards,
Roland