I added myself as a tester for my Android app. Received and opened the email on my Android device. Got to this screen:
Clicked on "Download" and got this:
The problem now: The "INSTALL" button doesn't work, i.e., it's not clickable.
This was on Nexus 5X Android 7.0.
Make sure there aren't any apps drawing to the screen like screen dimming apps (see: https://android.stackexchange.com/questions/69981/why-cant-i-press-the-install-button-when-installing-applications-from-unknown-s).