Hi Sharkboy,
Welcome to the Apple Community.
Wondering if you are using iPhone. another device, or a Mac computer to access iTunes.
and
if you have you updated / upgraded to the latest iOS or MacOS.
You may need to be signed in with your Apple ID before your can make a purchase
Try this Apple Support article:
If your payment method is declined in the App Store or iTunes Store
If needed:
You may want to access your Apple ID account, online to check the expiry date on your card.
https://appleid.apple.com
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If you still need help:
How to check if what you are trying to purchase is available in your country or region
Availability of Apple Media Services – Apple Support
How to check if your bank card is acceptable to Apple in your country or region
Payment methods that you can use with your Apple ID – Apple Support
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Also:
Apple support have another article online that may be helpful.
As you are unable to buy content:
If you can't open the App Store or iTunes Store, buy content, or edit payment information - Apple Support
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More info (Mac Computer):
Intro to the iTunes Store in Music on Mac – Apple Support
Set up and view an iTunes Store account in Music on Mac – Apple Support
iTunes User Guide
Welcome to iTunes on Mac – Apple Support
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More Support:
Eg: You may want to reach out to iTunes Support Team
iTunes Support Website:
iTunes - Official Apple Support
iTunes Support. Worldwide:
Choose your country or region - Official Apple Support
All the best :-)