apple card because it says not valid

Sign in to your Apple ID account page and edit your Apple ID details.

https://appleid.apple.com

On the Sidebar click Payment methods -> then tap Payments Methods and tap Manage payment method.

See also the following Apple articles:

Change, add or remove Apple ID payment methods

Change, add or remove Apple ID payment methods – Apple Support (UK)

Add money to your Apple ID balance

Add money to your Apple ID balance – Apple Support (UK)

Payment methods you can use with your Apple ID

Payment methods you can use with your Apple ID – Apple Support (UK)

See the Apple Support article(s) below. Link to contact Apple Support in the article. 

Apple ID payment information - If your payment method is declined, or you have a billing issue with a previous purchase in the iTunes Store*

Possible fixes.   Fix ‘Your Payment Method Was Declined’ Error In App Store.  -

                          Payment method declined in iTunes or the App Store.   -

See this Apple article:  If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card

If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card - Apple Support

See the following Apple articles:

Terms and Conditions for Apple Gift Cards and Codes, Apple Account Associated Balance, and Content Codes

Legal - iTunes Gift Cards and Codes - Apple

About Gift Card Scams

About Gift Card Scams - Official Apple Support

I still don’t understand why I bought card without used says it’s already used by another customer I was trying to check the balance show the balance $800 suddenly I can’t access to it again if I wanted to check my balance says invalid pin why I still don’t understand please help me find out everything I wanted to know what’s wrong and where I bought this card is far from me I call them they are doing what they like no good response from them so this is how I’m going to lose $800 I still don’t understand please help me

[Image Edited by Moderator to Remove Personal Information]