TL;DR: Open Android SDK Manager and install “Extras/Android Support Repository”.

After importing the unitypackage from play-games-plugin-for-unity, the project throws an error and stopped working. The error was not helpful at all, but the warning “Unable to find dependency” indicates that the “Android Support Repository” is missing. Even if the support repository is not installed, the folder “$SDK/extras/android/m2repository” is present.
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