Hello and thank you for your review.
Unfortunately, I am unable to refund your purchase on my end. To get a refund, you must contact Apple directly. They are the ones that can refund transactions. You can find more details about how to do it here: https://support.apple.com/en-us/HT204084.
If you want to use the free version of Decibel instead of the Pro version, you can re-install the app and simply skip the subscription screen that appears when you first open the app after download. The free version has many useful features for sound measurement and hearing protection and I recommend it if you are a casual user.
Hope my reply helps and wish you all the best!