Why can't I delete the payment method on my iPhone?

As the message says, you must first regularize your payment for a subscription that is still active.

For any further question / doubt, please contact the iTunes Store Support Team directly, using this link:--> iTunesStore & App Store – Get support for various issues, and choosing: your country, then Apple ID, then iTunes & App Store. If your problem is not among the options shown there, tap/click on the option “the topic is not listed” and describe your issue.

Regards

Giulio