What device (e.g. computer or phone) and connection (e.g. wifi or carrier data) are you using?
There is a known issue whereby stuff like this does not work when trying to do it from the phone over the carrier data connection when you have no data allowance left. In that case you need to try wifi or a computer.
As a stopgap measure until this problem is sorted you could topup your credit balance by voucher and the buy membership by text:
How do I top up? Overview of the top-up payment op...
How do I buy stuff by text?
Hope this helps.
