If this is a subscription app or a trial app, then you did authorize a charge by downloading the app.
You will need to cancel any subscription.
Managing auto-renewing subscriptions -
http://support.apple.com/kb/HT4098
Then ask Apple directly for a refund.
Get Support -
M1 Mac mini, macOS 11.1; Watch, series 5 GPS/cellular, watchOS 7.3.3; iPad Pro 2nd Gen, iPadOS 14.4.2; iPad Air 2, iPadOS 14.4.2; iPhone 12 Pro, iOS 14.4.2; TV 4th Gen, tvOS 14.3 HomePods, audioOS 14.3 and Airport Express w/ Airplay 2