Developers are unable to issue a refund for Apple iTunes subscriptions.

This official article will help you request a refund directly from Apple.

Note: You might be logged into a different Apple ID than the one you used to subscribe. Make sure you're using the same Apple ID and go through the steps in the article again.

Related articles
[iOS] How to cancel subscription in Ultimate Guitar iOS app or discontinue the free trial?  
[website] How do I cancel my Ultimate Guitar Pro subscription or discontinue the free trial?  
[Android] How to cancel subscription in Ultimate Guitar Android app or discontinue the free trial?