Unregistering a VM in Aria Automation fails with "Only onboarded machines can be unregistered".
Aria Automation 8.18.1
This is because the target VM is a Placeholder VM.
If the target VM is a Placeholder VM, it cannot be unregistered.
To remove a VM from Aria Automation management, ensure the target VM does not exist in vCenter, then delete the target VM from Aria Automation.