How to troubleshoot a Android building error

I was able to build my app for Android with 4.13, but the Play store rejected the APK, apparently because of safety issues with a PNG library. The library was to be replaced on 4.14, so I decided to wait for it, instead of building the engine from github like braver people did. Unfortunately, after updating to 4.14, I am no longer able to build the app:

UATHelper: Packaging (Android (ETC1)): Program.Main: AutomationTool exiting with ExitCode=5 (5)
UATHelper: Packaging (Android (ETC1)): BUILD FAILED
PackagingResults:Error: Error Unknown Error

Those are the last lines of the Output log. I don’t know what to look for on that log. I also tried to build it to ETC2 with similar results. Should I submit the rest of the output?

Any help is very welcome because I’m a total noob on all this. Thanks!

SOLVED! Key store file was missing in Project Settings > Platforms > Android > Distribution Signing > Key Store.