Hi Carolin, if you haven't already, you need to add your Visa card as a method of payment. To do this, you would go to "My Account" and then click on the tab that says "Billing Info." There, you can add a new method of payment (which in this case would be your Visa card details); after having done that, while still on that page, select that method of payment as the one that you wish to use to buy this game. If you don't wish to use your Visa card the next time you purchase a game, then you will need to go back to this Billing Info page and choose a different method of payment.
Please let me know if this does not answer your question or does not help you.