Apple music subscription payment issue

You'll likely need to enter a payment method which isn't declined. Not all payment methods are approved with Apple.

See this link --> If your payment method is declined in the App Store or iTunes Store - Apple Support

That's probably why it's declined. Your card is tied to your Apple ID for payment and using the same card for another users account, would likely be a red flag.

If you want to pay for an Apple Music Subscription for someone, why not just use Family Sharing --> Family Sharing - Apple