How can I report an app that contains ads but didn't provide a privacy policy url to Apple?
An app that doesn't have a privacy policy isn't necessary breaking any of Apple's rules or guidelines. The requirement that all these apps must have a privacy policy is quite new, and only apps or app updates submitted to the App Store after October 3rd 2018 need to comply.

You can use the following link to contact Apple's customer service:

https://getsupport.apple.com/

Choose "Apps and Software" and then "App Store".