Problem with mac mini app store

HI, im from argentina

i can purchase apps with my credit card in my ipad's app store but when i try to buy some app or soft in my mac mini's app store it brings a pop message that says:

"Your Purchase Could Not Be Completed, Please contact apple support"

and when i OK that, comes another message:

"We could not complete your request. There was an error in the App Store. Please try again later. (null)"

Obviously it has nothing to do with my credit card since in my ipad i can use it with no problem, any advise?

Thanks