If your app sells digital content or functionality that's consumed within the app itself — extra lives, premium features, digital currency — that transaction has to use Google Play's billing system rather than a separate payment processor or external link, with narrow exceptions.
A common cause is an app that started as a web product and kept its existing payment processor for in-app digital purchases instead of migrating to Play Billing when it was packaged as an Android app.
"Your app offers users the ability to make in-app purchases of digital content using a payment method other than Google Play's billing system, per the Payments policy."