There have been a few changes recently that will require you to update your IntelliJ Gradle Config. You can manually synchronize but I suggest enabling "Use auto-import. Now is also a good time to make sure you have "Create separate module per source set" enabled as well.
Please also make sure you have the latest IJ installed, 2018.2.2. I recommend that you install the JetBrains Toolbox and let it manage and keep you IJ auto updated. -Jake
