Apple has released iTunes 12.8, for Mac and Windows, adding AirPlay 2 support to the desktop. When Apple released iOS 11.4, with AirPlay 2, the Mac (and Windows) was notably absent, meaning that you could stream music to a stereo pair of HomePods from an iOS device, but not from iTunes. This his (finally) been corrected.
Download iTunes 12.8 here, or through the Mac App Store app.