To try Apple Music the card on your account has to be accepted by the iTunes validation - the card may work on other sites, that doesn't mean that it will work with iTunes.
What are you trying to use as your payment method, credit card, debit card ... ? If it's a debit card then not all seem to still be accepted, and might not be accepted at all in some countries.
For a card to have a chance of being accepted it needs to be registered to the same name and address (including format and spacing etc) that you have on your iTunes account, and have been issued by a bank in the country where you and your iTunes account are.