Unfortunately there are a lot of people experiancing this and other problems when trying to buy simpoints. Some get the card not working error. I get the "a system error has occured" and haven't been able to buy simpoints for over a month.
On the official forum there are a number of threads disgussing this issue. It seems EA is aware of the problems wbut still haven't fixed it (this has been going on for several months now). I would suggest you try to contact EA support since some people have reported that that helped. (Not in my case unfortunately).
Also, if you are in the US you could just buy from origin instead - their system if working. Unfortunately their prices are more expencive if you aren't buying from the US.
Also, using paypal or a different browser also could help out.
I would be really interested in finding out how things work out for you.