Assuming the item that you are attempting to obtain refund is very recent, you will need to wait for the related purchase invoice to arrive from Apple via email - this usually arriving with 24 hours. While the charge is pending, you canʼt request a refund; after the charge goes through, try to request a refund again.
In many countries/regions, App purchases are final. However, within the invoice email will be a “Report A Problem” link - where you may request that an App purchase is cancelled. Apple, at their absolute discretion, may cancel the purchase and make a refund to the registered Payment Method that was used to purchase the App.
Where a refund request is permitted, you have a limited period of time within which to make the request.
More information about requesting refund for unintended App purchases can be found here:
Request a refund for apps or content that you bought from Apple - Apple Support