Payment

Hi reva96,

Welcome to the Apple Community, where some of us do our best to help each other.

Apple Support have an article that will answer "Why". 

"If your payment method is declined in the App Store or iTunes Store”

https://support.apple.com/en-au/HT203005

Apple ID is the account used for everything Apple, including iTunes, iCloud, Music, etc.

The link below is to an article from Apple Support:

You can find your country or region to find out what payment methods are available

Payment methods that you can use with your Apple ID

https://support.apple.com/en-au/HT202631#methods

Your Apple ID account is where you can add and remove payment methods.

Here is the Apple ID LogIn:

https://appleid.apple.com/

Here is how to:

Change, Add or Remove Apple ID Payment methods

https://support.apple.com/en-us/HT201266

Hope something here will help you doing what you want to do. :)