Ah, this won’t work. The default dmengine.apk which you can download from GitHub and d.defold.com does not include the IAP extension since it has been removed from the engine. You need to run a version of Defold which includes the extension. This is mentioned here:
“The stand-alone version will only work for projects without any native extensions. If your project contains native extensions you must bundle a Debug variant of your project to ensure that the dev app contains the native extensions that you intend to use.”
So bundle your game once using Project->Bundle->Android. Make sure you bundle a debug build. Install and start it. You will now be able to target it and have access to IAPs.
PS: Note this post about bundling issues in 1.2.173: Problem bundling APK/AAB file from editor in 1.2.173