There are few options for you to Top-up.
If the system detected your balance is insufficient, it will redirect you to Top-up your balance.
Select Payment Method
Once the Top-up successful, you have to insert Setel verification PIN and the order will be processed.
Otherwise, you may also Top-up your balance in the Setel app itself:
You need to log in to your Setel App, go to More > Payment Methods > Top-up