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. After having a look at the logs your issue seems the same.
This solved my issue with failed packaging - >
Go to - NVPACK\android-sdk-windows\tools\android.bat
Downloaded/Update Android Support Repository and Android Support Library
It should be at the bottom
Package again and it will work. Assuming everything else is set up properly.