There can be a couple things. There is a spending limit that you can set, you may have exceeded that.
Some people also have had trouble making more than one purchase in a 24 hour period with the same credit card but have managed it on 2. I'm pretty sure that it isn't supposed to do that but I don't know if it's been fixed or not.
Either way I believe 24 hours after your last purchase should allow you to purchase again.
Unless of course I'm completely wrong, which is certainly a possibility. 
Thats what I thought. But I set my spending limit to $240 and my last purchase was almost a week ago. And the error I get is.. "We're sorry, but we are unable to accept your order at this time. Please try again later or tomorrow. If you continue to have problems, you may wish to visit one of our retail partners."
The Store was down today and probably was just acting wonky still. Give it a shot tomorrow.
Dec out.
This might be in the wrong spot but. I just need to know whats the timeframe restriction on buying things in the NCSoft Online store? I'm trying to buy Going Rogue and I keep getting an error.