Wi-Fi Connect Failed, possible cause MAC address filtering
If your router or wireless access point is set up to do MAC address filtering disable the feature before attempting to set up your HomePod. I forgot, resulting in the Wi-Fi Connect Failed error and had to power down the HomePod in order to run through the set up again.
Once your HomePod is set up you can find its MAC address in your list of connected devices, add it to the approved list and enable filtering again. If your router is anything like mine changing these settings kicks everything off the network adding to the effort involved.
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HomePod, Space Grey