How to reproduce:
1. Create a new project
2. Switch platform to Android
3. Keep everything default, just change Bundle Identifier
4. Build Empty project on Android

Result: Build fails with an error UnauthorizedAccessException: Access to the path '/Applications/Unity 2018.3.0a4/PlaybackEngines/AndroidPlayer/Tools/gradle/lib' is denied.

Note: If you go to provided folder you can notice that "lib" folder cannot be open

Reproducible: 2018.3.0a4 (09d093d4c7ae)
Not reproducible: 2018.3.0a3 (0ac1bca97520)

Reproducible (tested and reproduced): Macbook Pro OSX 10.13.5; Macbook pro OSX 10.13.4
Not reproducible (tested but didn't reproduce): PC Windows 10 Pro

Workaround: use Internal build system