Hi m4rkvx,
welcome to the Apple (user-to-user) Community!
Re: "How to fix error "iTunes Store is unable to process purchases at this time.Please try again later.”
As Apple users, like yourself in this community, we have no access to your accounts, details, or purchase history.
We'd like to help, however.
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Re: "How to fix this immediately?"
Seems we need to find the cause of why you have this issue: eg: could be any one of several situations.
You may want to check you are signed in to your iPhone with the correct Apple ID you use for iTunes purchases:
ie: See how to: Sign in to your Apple Account - Apple Support
You may want to check there are no outages: Apple - Support - System Status
Also, you can check to see if any restrictions are set on purchases:
See how to: Manage App Store purchases, settings, and restrictions on iPhone - Apple Support
Check that the payment method you use is one that Apple can accept in your country or region.
Payment methods that you can use with your Apple ID - Apple Support
You may need to check that your financial institution allows payment for International purchases to process.
You can check your payment method has adequate funds, or card expiry-date has not expired, etc:
Eg: You can Log-in to your Apple ID account online anytime you wish. It is wise to ensure your details are always up-to-date. (You can check, add, or edit your details, if you wish.)
Apple Account online sign-in: https://appleid.apple.com
If needed: You may have to add a new payment method before you can remove an old one:
What to do: If you need to change or update your Apple Account payment method - Apple Support
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If you still need help: Apple Support has trouble-shooting articles online:
What to do: If you can't connect to the App Store, iTunes Store, or other Apple services - Apple Support
If necessary: Find out how to pay an unpaid order.
Eg: See: If a message says 'billing problem with previous purchase' or 'verification required' - Apple Support
or
How to: View your purchase history for the App Store and other Apple media services - Apple Support
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If all else fails to find and rectify the cause of the issue: Contact Apple Support
All the best :-)