Common reasons due to which a Virtual Machine creation fails in HC using VmWare / Hyper-V Modules

Answer:
 There could be several reasons but following ones are the major reasons for VM creation failure:

Reason 1: 
The vmdk/vhdx file is corrupted. You can ensure this by navigating to the path: Configuration:: Virtualization Module Configuration:: OS Templates:: Inspect Disk, here check that the inspect disk should return values. If it does not return any details about the vmdk/vhdx file then you would not be able to create VM through the HC panel.

Reason 2: 
The size specified for Virtual Hard Disk while adding a Virtual Machine from HC panel at Provisioning:: Virtual Machines:: Create Virtual Machine is less than or equal to the Maximum Size for OSTemplate mentioned at Panel Configuration:: Virtualization Module Configuration:: OS Templates:: Inspect Disk against “Maximum Size” field. In this case, the VM creation would fail through the HC panel. Specify size greater than the maximum size for OSTemplate. 

Reason 3:
If the Master OS Template(Guest OS) whose vmdk/vhdx file is used for Virtual Machine creation is Powered On, then in this case also HC would not be able to create any Virtual Machine. You would need to shut down Master OS Template machine and then try creating VM through the HC panel.

Reason 4:
If the OS Template Default Admin Password is not specified correct at : Panel Configuration :: Virtualization Module Configuration :: OS Templates :: Edit OS Template from host level in HC panel then VM creation would fail through HC panel.