Did you change something else than just upgrading to 4.26? We’ve been using 4.26 for a while now, but still get the error from Google. I even tried 4.27, but still get the same error. There’s also not much documentation aside from that one XML mentioning the Billing library version for Gradle. Which Gradle version are you using? We have 6.1.1 from when we switched to UE4.25 with the recommended new Android setup.