Unable to complete payment for app subscription

Re: "Unable to subscribe : Failed to complete the payment when purchasing the app"

Your Apple Account is the account you use to access Apple services like the App Store, Apple Music, iCloud, iMessage, FaceTime, and more. It includes the email address and password you use to sign in as well as the contact, payment, and security details you use across Apple services.

As Apple users, like yourself, in the community, we do not have enough info. to simply troubleshoot the situation. Apple support have various articles online (links below) that may be helpful:

Check you are signed in to your iPhone with the correct Apple Account details you use for prchases:

If needed: See: Sign in to your Apple Account on the web or in your device settings - Apple Support

You can check which payment methods Apple can accept in your country or region:

Payment methods that you can use with your Apple Account - Apple Support

If needed: See how to Add an Apple Account Payment method:

Add a payment method to your Apple Account - Apple Support

See what to do: If your payment method is declined in the App Store or iTunes Store - Apple Support

or

See what to do: If you can’t download or update apps on your iPhone or iPad - Apple Support

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If needed: More info. / support / contact options etc.

Billing and Subscriptions Support website: Subscriptions and Billing - Official Apple Support