Is it mandatory to do an online challan payment

It's usually required to make challan payments online. For things like income tax, you absolutely need to create the challan online first. If it's a traffic e-challan, you can often pay with a card when it's issued, or you can pay online later using a credit or debit card. It's a good idea to pay these within 15 days, or else a report might be sent to court.