How do I remove an active mobile phone subscription from my Apple Account?

I am confused by your question. First you say you want to remove a subscription from your account, then you describe a scenario which instead indicates you are trying to possibly remove a payment method. Which is it?

This support article explains how to cancel a subscription from Apple on an Apple mobile device, Mac, PC, and online (see the Android section) ➔ If you want to cancel a subscription from Apple - Apple Support

You cannot remove the only payment method (account balance does not count as a payment method in this scenario) if you have an active subscription. "Active" means even one that you have canceled but has not yet expired fully. You can only update a payment method in this scenario and a new one has to be added first before removing the old one.

See this article for information about updating payment methods on various platforms (iPhone, Mac, etc.): If you need to change or update your Apple Account payment method - Apple Support

You can’t remove a payment method from your Apple ID (including a mobile phone billing option) until ALL active subscriptions that depend on it are changed or canceled.

This is why the “Remove” button is grayed out and you see the message:

“Active subscription — cannot remove payment method.”

Hello~ Take a look by clicking on below as it outlines your issue…

Remove a payment method from your Apple Account - Apple Support

~Katana-San~