ok. Can you confirm you followed the suggestions above? Restart, re-install, log out of App Store?
can you verify that the Apple ID you are using now is the same that you used to originally purchase the app?
what iOS version are you using? If it's an old version, can you try and upgrade?
Do you happen to have an Apple developer account yourself, and possibly used your Apple ID in the sandbox development environment? This can mess things up with the App Store, it has done that for me in the past.
Tbh I'm just making wild guesses here - purchase processing is handled by Apple, not by me. If you can't restore the purchase, it means that the Apple server tells STROM that there was no previous purchase from the currently logged in Apple ID, and that's the extent of information I'll receive.
please follow the instructions here rigorously: https://support.apple.com/en-us/HT204530
especially double check that you have the IAP listed in your purchases. Hope that'll sort it out.. cheers!