Hello @mgbrown,
As the VM instance has been deleted, the disks are not at all associated with the VM and the command can't work. So, just try Get-AzDisk and find the unattached disk and delete manually. Or please find the Microsoft documentation explaining the way to delete it.
https://learn.microsoft.com/en-us/azure/virtual-machines/windows/find-unattached-disks
Next time, I am suggesting yo to refer my blog, in which I have an article to handle this situation. Copy the script as .ps1 file and execute it in PowerShell. Just input the VM name and the script can clean all the resources associated. The script creates necessary labels and attach to the disk before the VM deletion, and delete works well by referring the special labels.
http://cloudcompute.info/delete-azure-vm-and-all-associated-resources-using-powershell-script/
Please mark as "Accept the answer" if the above steps helps you. Others with similar issues can also follow the solution as per your suggestion
Regards,
Manu