Hello
Since you mention helper VM, I assume the question is about Linux P2V. If that's true, then the error message is misleading. It is being thrown at a quite general place, so this is possible.
However, Converter worker creates the destination VM and partitions its disks. It is possible that some permissions are missing in this respect.
Pls attach the logs (the worker log to be more specific) for more adequate analysis.
Regards,
Plamen