Hi there. Many thanks for your reply. This randomly happened to me today in a project I was able to package just fine previously. Although I had to use implementation 'com.google.android.gms:play-services-games:22.0.1' instead of the one you provided, your answer still lead me on the right track. So, again thank you very much.
In case anyone else comes across this issue: You can find the correct version to put in the command in the build.gradle file from the engine directory (…\Engine\Build\Android\Java\gradle\app).
Cheers