Can you share details of conversion?
Or upload log bundle for investigation.
Can't you just go into the VM's properties and change the virtual hardware version?
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\
If its a Windows VM try disabling Firewall from inside windows OS.
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.
Fastest way is create another VM in hardware version that you require, and attach the VMDK files of your old VM to this new VM.
If there is any error, then V2V converting by VMware vCenter Converter is a safe way, but if you can first convert it to VMware Workstation type of VM and then convert it back to the ESXi type of VM. (I did something like this many years ago)Please mark my comment as the Correct Answer if this solution resolved your problem