Check minimal API level, look bottom of this page:
API in a increment number that increases each time API changes in Android updates, so each Android version have it’s API level. Here you go list of API levels:
https://source.android.com/setup/start/build-numbers
You can set it even to 1 but won’t magically make UE4 run on older devices, it’s simply preemptive compatibility check. With ES2 with older UE4, I was manage to run UE4 on Android 4.1 so around level 16, so you can get quite low, if you got some old Android devices, run some tests and check.
If you still have a problem analyze manifest file (i think you can access it in project settings, or in your project directory in build folder):