How can I delete a subscription if it won’t let me

If you see ‘itunes.com/bill‘ or an unfamiliar charge on your bank, credit card, or debit statement - https://support.apple.com/HT201382 - "Before you contact Apple to request a refund for a charge you don't recognize"...

Subscriptions and purchases made by family members under Family Sharing (if applicable) will not show in your list or purchase history, you need to check these from devices using their Apple IDs.

If neither you nor a family member (if applicable) has a record of this purchase and you only saw it on a statement for a card you alone control, contact your bank or card company about card fraud. Apple does not handle that.

I have never heard of it not letting you, so tell us more please.