There are a few things which need to be considered, before making the final payment:
1: For an online payment to be successful, please make sure your card details match your billing details entered at the payment page.
2: Make sure the amount to be paid does not exceed the amount available on the card.
3: Contact your bank to make sure it has authorised the card to be used in online transactions.
If you still encounter difficulties when trying to pay online, you can raise a technical ticket and report the problem in detail (any screenshots or copies of error messages would help us identify the issue quicker).