Did you cancel it more than 24 hours before it was set to renew? If not, Apple indicates it may still charge one last time before actually cancelling.
How do you know the charge is for that specific subscription?
Does the charge appear on your payment method's statement with the name of the app / website or as "apple.com/bill"?
If it appears as "apple.com/bill". the charge may be for something else entirely.
If it appears as the name of the app / website, then it is being charged directly by them, and not Apple, and as such would need to be cancelled through their website directly.
If it does appear as "apple.com/bill" see the following support document to identify it and request a refund for it if possible. If it's a different subscription, cancel it before requesting a refund.
click here ➜ Get help with charges from apple.com/bill - Apple Support
If it is for a cancelled subscription, that you simply cancelled too close to renewal, then simply ask for a refund in the link below.
click here ➜ Request a refund for apps or content that you bought from Apple - Apple Support