If your account is registered in Belgium and your card is from that country too, you shouldn't have a problem to subscribe with it. Could you try to subscribe again at spotify.com/premium. We suggest that you try it from a computer, using a Google Chrome incognito window.
If the card you're trying to use is from Spain, you won't be able to use it to subscribe because the country of your payment method needs to match the county in your account. In that case, you can manually change the country on your account to Spain before trying to subscribe again.
Let us know how it goes or if you have questions. Have a great day.