Are you talking about this app?
I purchased this app and it does not work.
1.
If you have received an invoice from Apple, on every invoice from the Apple Store and/or iTunes Store always states what you have purchased (and/or concerns a subscription) and the amount.
Tap on Report a problem on the invoice.
Or try to -> Request a refund for apps or content that you bought from Apple.
Request a refund for apps or content you've bought from Apple – Apple Support (UK)
If you have requested a refund, you must wait until Apple has approved. Then you will receive a message from Apple.
Refunds are not guaranteed. For more information, see the Terms & Conditions.
2.
If you have received an invoice from Apple, tap on Report a problem on the invoice.

Then you can choose an option:


OR see -> Find the URL for an app in the App Store to report a concern
Find the URL for an app in the App Store to report a concern - Apple Support