You may be speaking to the HomePod, but the phone is the thing actually running the Shortcut. And the list of Shortcuts actions does not include controlling the

You may be speaking to the HomePod, but the phone is the thing actually running the Shortcut. And the list of Shortcuts actions does not include controlling the music output of a HomePod. I would encourage you to file a bug report with Apple at https://bugreport.apple.com https://bugreport.apple.com to request the ability to instruct the HomePod to play music. As for the HomeKit scene, that sounds like a bizarre error to get from a Shortcut.