As you can read here, Android 7.0 'Nougat' introduced the APK Signature Scheme v2, a new app-signing scheme that offers faster app install times and more protection against unauthorized alterations to APK files. This workaround should be the first thing to try if you're using older phones and/or Android builds <= 6.

Unsupported v2 Signature (aka Full-APK Signature)

Here's a couple solutions I've come with, mostly thanks to the following StackOverflow thread. APK and/or uninstalling the app's previous version (if any) seems to fix the issue. It also goes without saying that neither rebooting the phone and/ or removing the existing. That's not the best documented error you can hope for, isnt'it? What makes things ever worse is the fact that the APK is most likely working on other devices, including the Android emulator you (or the developer) just tested it with. Are you an Android developer or tester? Do you often play with APK files coming from 'insecure sources' such as signed APKs built with Android Studio? In case you do, you might likely experience the following problem when trying to install those apps:

Android studio apk for android