Why isn't Corber building an apk for my Ember project?

I want to turn an existing Ember project into a mobile app, so I decided to learn Corber, which links Ember and Cordova to (theoretically) make the webapp -> mobile transition easy. However the steps from Corber's docs give me the error 'No apk found'

I created an empty Ember project using 'ember new test-corber' then ran 'corber init' and chose android when asked what platforms to init with. I then changed the two environment variables that the Corber guides specify for Ember setup (and nothing else) and ran 'corber start'. At this point I always get the message "No apk found" and the process ends. The apk is supposed to be put in test-corber/corber/cordova/platforms/android/outputs/apk/debug/ but there isn't even an outputs/ folder.

I'm on Ubuntu 16.04, java 1.8.0_191, corber 1.3.10, gradle 5.4.1, node 6.16.0, and ember 2.18.2 (though I also tried it on 3.9.0)

I've tried selecting different emulators, reinstalling Gradle, creating a new project, and searching online for solutions to no avail. I'm open to any ideas or suggestions.