How can I tell whether the problem is with the device and not the app?

If Android stops the installation before our app even starts, the cause is usually the device, its security features, or policies — not the app itself. Typical clues are messages about:

• “Install unknown apps”
• Play Protect
• Advanced Protection
• admin or company policies
• package conflicts 

Another strong indication: the same APK works on another compatible device or the Google Play version can be installed while the APK version is blocked on that exact device. In that case, almost everything points to a system-level block on the affected device.