As all payments in Android are handled by Google Play, you should send your refund request directly to Google Support
This official article will help you request a refund from Google.
Note: You might be logged into a different Google Account than the one you used to subscribe. Make sure you're using the same Google Account and go through the steps in the article again.
Related articles
[Android] How to restore a subscription and favorites in Ultimate Guitar app for Android?
[Android] How to cancel subscription in Ultimate Guitar Android app or discontinue the free trial?