Hi ItsTashhh
Re: Unable to add card as payment for App Store
You can check your details are all correct and up-to-date, (or make changes), in your Apple ID account online:
Apple ID Log-in
https://appleid.apple.com
A verification code is sent to your trusted device:
Get a verification code and sign in with two-factor authentication - Apple Support
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Else:
Wondering if these Apple Support articles can help:
Check what payment methods you can use in your country or region:
Payment methods that you can use with your Apple ID – Apple Support
How to add an Apple ID payment method:
Add a payment method to your Apple ID - Apple Support
How to delete an Apple ID payment method:
Remove a payment method from your Apple ID - Apple Support
How to change an Apple ID payment method:
If you need to change or update your Apple ID payment method – Apple Support
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Re: cannot download anything in AppStore without the pop up of ads your card details.
Check you are signed in (to your iPhone) with your Apple ID
Sign in with your Apple ID – Apple Support
What to do: If your payment method is declined in the App Store or iTunes Store - Apple Support
or
If you can’t download or update apps on your iPhone or iPad - Apple Support
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if needed:
You might want to reach out to the App Store Support Team:
There's a "Get Support" link at the bottom of their webpage.
(you may get to choose from online chat, phone or Email etc.)
App Store - Official Apple Support
All the best :-)