Sometimes in the provisioning process VMs may get stuck or error out. While you could delete them directly from vCenter, this is not the best practice. Go to Infrastructure | Machines | Manage Machines.
From this screen, you can delete any VMs in the purview of vRA.
vRealize Automation deletes failed VMs after a period of time and makes troubleshooting difficult. Be sure to read Chapter 11, Troubleshooting vRealize Automation 6.2, where we illustrate how to disable this feature so you can analyze why the VM provisioning failed.