Hello FionaPaul,
After reviewing your post, it sounds like you are trying to change the payment method, but it is not letting you because you have an active subscription. I would recommend you use this article to help troubleshoot the issue.
"View your subscriptions
If you have any active subscriptions, you must have at least one payment method on file. If you want to continue enjoying your subscriptions, you can change your payment method. But if you want to remove all payment methods, you can cancel your subscriptions and/or downgrade your iCloud storage plan. Before you do this, you might want to make a backup of your devices without iCloud.
When your subscription period is over, try to remove your payment method again."
If you can't remove your last payment method or use no payment method with your Apple ID
Please don't hesitate to let us know if the steps resolved your issue or if you need additional assistance.
Thank you for using Apple Support Communities. Have a nice day.