Why does my Internal Test Track opt-in URL not work?

I'm developing an app and am ready to release it to a small group (5) of testers for internal testing, so I added their Gmail addresses to the list:

I sent one of the testers the link to the app and after opening email and selecting the link, they got this:

followed by this:

So far, so good. Now when I select the tester in question, instead of advancing to the next page, which is the start of a tutorial (as the app does when it's run from Android Studio), it returns to the previous screen. No error message or crash (I checked with Crashlytics). Anyone have any idea why this is happening?