With Gradle you need to have the Android Support Repository and Android Support Library
I was having problems with packaging too, but I realized that I was missing a repository. Though the editor does give out a message about this and packaging without grade should work fine at the present state. Your problem seems like something else. Still, it is worth a try…
This solved my issue with failed packaging - >
Go to - NVPACK\android-sdk-windows\tools\android.bat
Downloaded/Updated Android Support Repository and Android Support Library
If you are packaging with a C++ or nativization enabled project. Make sure you have Visual Studio Community 2017 well set up
If not try packaging a blank blueprint project and see if it successfully packages. If yes, then it is some project specific issue.