Hello There,
Sol-1:
Have you tried powering off VM and then v2v ? Please power off the VM and then try the same task. Also you can analyze logs under below path on machine where you have installed VM Converter for clarity of errors.
C:\ProgramData\VMware\VMware vCenter Converter Standalone\logs\
Sol-2:
If its a Windows VM try disabling Firewall from inside windows OS.
Sol-3:
The account which you are using for v2v must have administrative privileges on the VM which you are trying to convert.
Although Lowering down H/W Version is not recommended, but you can do it by v2v as you are doing it. For first you can try v2v after powering off your VM.