If you don't show up for your activity, or you arrive after it has left, no refund is possible.
Important
To avoid a no-show: note the time and meeting point listed on your voucher, allow extra time for the unexpected (transportation, lines), and arrive early.
If you think you won't be able to make it, check whether your booking can be canceled and cancel it before the free cancellation period ends: you can then be refunded according to the activity's policy.