Can you try another browser and/or device?
If that doesn't work then you'll probably have to contact 48 support:
How to get help
Several people reported problems with the payment/purchase pages recently so maybe the problems persist?
In the meantime you could alternatively topup by voucher and then buy stuff online or by text from your credit balance:
How do I top up? Overview of the top-up payment op...
How do I buy stuff by text?
Hope this helps.
