How to own apple tv when iam adding payment details its getting declined

Payment methods are for your Apple ID account (not for a specific product item), to be accessible continuously when you want to make an iTunes Store purchase or App Store purchase. Like an open tab. Subscriptions are a bit more stringent for payment method, as subscriptions require processing of automatic and periodical charges. A credit card usually works most convenient.

Payment methods that you can use with your Apple ID - Apple Support

Change, add, or remove Apple ID payment methods - Apple Support

You may add more payment methods as fallback.

If one payment method is declined, try to find out why. If your payment method is declined in the App Store or iTunes Store - Apple Support

Apple ID balance is also an option, by adding funds to your account first, then any charges get deducted from there.

Add money to your Apple ID balance - Apple Support

Check your Apple ID balance - Apple Support

What you can buy with your Apple ID balance - Apple Support