Error when trying to buy coins

I'm trying to buy coins so my kids can get a texture pack and when I click on the icon to get more coins it pops up the error (see screen shot): "we were unable to complete your purchase. Maybe check your internet connection?" Two weeks ago I was trying to buy coins and the same error popped up. I have not yet been able to buy any coins. There is nothing wrong with my internet connection.  Any suggestions would be great as my kids would really like to buy these additions to Minecraft.