It appears that you're unable to use your credit card as a payment method for your Apple ID in Belgium. I'm happy to help.
Payment methods that you can use with your Apple ID
Some countries and regions can use PayPal or mobile phone billing (through supported carriers).
If you change your payment method to PayPal, you might be asked to verify your account. If you are, you'll receive a verification code either through a text message to the phone number on file with PayPal or instructions to get the code through a code generator. If you can’t access this verification code, you might need to sign in to PayPal to update your phone number or contact PayPal for more help.
PayPal is only available in Australia, Austria, Belgium, Canada, Finland, France, Germany, Greece, Ireland, Israel, Italy, Luxembourg, Mexico, Netherlands, Portugal, Spain, the United Kingdom, and the United States.
Mobile phone billing is only available through supported carriers in select countries and regions.
Learn how to set up mobile phone billing.
Set up mobile phone billing as your Apple ID payment method -- If your wireless carrier is BASE-PayByMobile or Proximus, and you'd like to use this method, follow the steps provided in this article.