Reproduction steps:
1) Create a new project
2) Go to Build Settings (don't forget to change bundle identifier in order to build successfully)
3) Select Gradle as Build System
4) Select "Export Project"
5) Change Build System back to Internal
-- Notice that "Export" button remains

Regression introduced in 2018.1.0a1
Reproduced with: 2018.1.0a1
Unable to reproduce: 2017.3.0b5, 2017.2.0f3

If you press the Export button, Unity will build APK file but without ".apk" extension in its name. If you add ".apk" extension manually you can install that application on Android successfully.