I need to cancel a purchase

This is primarily a user-to-user support forum; other Apple users like you. We cannot cancel anything but the following may help.

See instructions in: Request a refund for apps or content that you bought from Apple - Apple Support

Also see the following from that link:

If you or a family member bought an item by mistake, set up Screen Time to prevent unintentional purchases in the future. You can require a password for purchases, prevent some types of purchases, or disable purchasing completely. Set up Screen Time.

If it was a subscription, also see: See or cancel your subscriptions - Apple Support

Start here: https://support.apple.com/en-us/HT204084