Problem with StudioOne 3 discount

So recently I wanted to upgrade from StudioOne3 Artist to SO3 Professional , but the problem is when I select discount from product that I wanted to buy but it don't let me go to payment screen. I don't really want to spend full price for it when the side say that I can upgrade from version and at the end i can't buy it. Sorry for my bad English...

Cheers!

Konrad