How can I get a refund for a subscription?

Subscriptions aren't usually refundable but you can try the following. There is an Apple article with information about requesting refunds. Click on the following link: Request a refund for apps or content that you bought from Apple - Apple Support

You don't get money back after cancelling a subscription. It just will no longer renew and you will continue to have the service you have paid for until that time. That is how all subscriptions work. The only thing you can do is try the refund link provided already. Apple will at times offer a compassionate refund, but it should never be expected. Forgetting to cancel a subscription is not likely going to be a valid reason for a refund.