I have Windows 10 working. I am not sure which of the actions I took made the difference.
When I attempted to start up the already installed VM, this time I got the error that it could not find a valid bootable disk. So I deleted the virtual machine, and exercised the option to delete all associated files when doing so. I then updated the my Ubuntu OS using the "sudo apt update", and "sudo apt upgrade" commands to cover the possibility there would be a fix that would make it work in the upgrade. I also went to Discover, and updated all installations there.
In VirtualBox, when I selected the file "Win10_21H2_English_x64.iso" I checked the "live CD" box. I had always ignored this box before.
Then when I started up the VM this time it proceeded with installation.