Purchase couldn't be completed

I have money on my card and I'm trying to spend those for a limited offer on a game which is gonna end tomorrow !!! iTunes doesn't let me buy 20$ deal, I dunno why! I could buy the 10$ one but I'm stuck to "purchase couldn't be completed contact iTunes support" there's nothing in the support that can help me! I tried to contact them via mail but I didn't receive any answer. Please help me fast!!! Thanks in advantage

Sincerely

Kira