I want a refund on a transaction that took place with a canceled subscription

Click on this link --> Request a refund for apps or content that you bought from Apple - https://support.apple.com/HT204084

If you used Apple Pay you can also perform these steps

If you see a transaction you do not recognize or have an issue with a transaction open Wallet>tap on the card used>tap on the transaction (photo 1)>tap on the amount (photo 2)>look for a report an issue link. (photo 3)

photo 1 - select the transaction

photo 2 - click the amount

photo 3 - click the link