Encountering the same issue, even when trying to install the APK directly from APK Mirror
APK Mirror shows all app versions from the past year requiring Android 6.0+, while Fire tablets are built on Android 5.0 (except for the Fire 8 released in Nov 2018). So I think that's the issue and not sure we'll be able to overcome that until Amazon released a new suite of Fire tablets or upgrades old tablets (unlikely)