How to request a refund from App Store

I bought discord nitro for $10, and it showed that I was all set but next thing you know it shows that an error occurred and now I don’t have it. I tried to cancel the subscription AND restore it but now apple is charging me for something I did not yet receive. It’s also $10 for 1 month but it’s charging me to pay $19 for some reason? I need help or a refund

[Re-Titled by Moderator]