Thank you. I’d looked over that page already. It doesn’t really address this issue, but it looks like for stand alone VM’s that if they do not have a Qubes agent installed, it can’t interact with Qubes salt, and the upgrade doesn’t progress any further. Since I had a backup already, I removed those VM’s and restarted the upgrade, and it then succeeded.
This is almost true. There must be some salt agent for any qube to be
controllable by salt. Using the Qubes agent is the simplest solution,
where possible.