What happens?
Where are you trying to do this - on the iPhone app or on the website?
If possible can you try the website and maybe even another browser if it also fails in your regular one?
Are you trying to use the card already registered on your account that was successfully used to make payments on 48 before?
Or a new card that was never used online or on 48 before?
If necessary contact 48 support for advice:
http://www.48months.ie/help#ask-a-48-agent
In the meantine you could possibly topup by voucher (if you are not already abroad) and buy stuff by text:
How do I top up? Overview of the top-up payment op...
How do I buy stuff by text?
Hope this helps.
