"Your subscription still renews. Cancel it under Billing first; you can delete the account afterwards."
We stop you deleting the account while a plan bought on the website still renews, because after deletion you couldn't sign in to cancel it any more.
- Open Billing → Manage billing and cancel the subscription.
- Go back to Your data and delete the account.
Once a subscription is cancelled ("cancelled, runs to the end of the period"), you can delete the account straight away, even if the paid period hasn't ended yet. Remaining paid time is lost with the account.
An App Store subscription doesn't block deletion, but it isn't cancelled by it either: cancel it in the iPhone's Settings → Apple ID → Subscriptions.