how to cancel payment method. Not interested in subscriptions.

See -> Remove a payment method from your Apple ID

Remove a payment method from your Apple ID - Apple Support

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If you can't remove a payment method

If you have subscriptions like iCloud+, have an unpaid balance, or you use purchase sharing, you must keep at least one payment method on file.

• Do you have a subscription? You can cancel iCloud+ and cancel other subscriptions from Apple. When your subscription period is over, remove your payment method.
• If you're in a Family Sharing group and you use purchase sharing, the family organizer must have a payment method on file. If you're the family organizer, turn off purchase sharing, then remove the payment method.
• If you have an unpaid balance, you might not be able to remove a payment method. Instead, change your payment method.
• 👉 If you still can't remove a payment method, contact Apple Support.

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Not interested in subscriptions.

FYI:

You can download free apps, but you may need to have a valid payment method on file, even if the app you want to download is free. 

Free apps has also In-app pruchase.

See -> If you can’t download or update apps on your iPhone or iPad

If you can’t download or update apps on your iPhone or iPad – Apple Support (UK)

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Add a payment method

You may need to have a valid payment method on file, even if the app you want to download is free.

Add a payment method

If you still need help, get support from Apple directly -> Get Support or call Apple in your region see -> Contact Apple for support and service