I got an email for undone purchase for Roblox. It is done through itunes. How should I react?

Follow the steps in the support document you posted from if you require a refund and the purchase actually exists on your account. Request a refund for apps or content that you bought from Apple

If you just received an email, but there is no charge on your account, or your payment method's statement, then you can safely ignore it and consider it a scam or phishing attempt.

Link-> Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

Link-> Identify legitimate emails from the App Store or iTunes Store - Apple Support