I can’t delete my bank details from my Apple ID

Do you mean you canceled your subscriptions? Subscriptions will run until the expiration date. Canceling them just tells them not to renew after that date. Until then Apple will not let you remove a payment method that is

being used to back any subscriptions you have going. Refer to:Remove a payment method from your Apple ID - Apple Support

To have it end before that date you need to contact Apple and explain you need it to end immediately.

Click here --> Choose your country or region - Official Apple Support Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service --> "See a list of Apple phone numbers around the world." Click here --> Contact Apple for support and service - Apple Support

You will need to have at least one payment method on file. If you have that ,and you still cannot delete your bank card, Contact - Official Apple Support For further reference