unmap task failed: Cannot complete the operation due to an incorrect request to the server

Hi,

I implemented the unmap task through vSphere 6 api method: UnmapVmfsVolumeEx_task(HostStorageSystem, VmfsUuid).

But when I run this task on vsphere web client side, this task failed:

TaskName: host.StorageSystem.unmapVmfsVolumeEx.label

TaskStatus: Cannot complete the operation due to an incorrect request to the server

I try to get the task info and the esxi log files, however no more useful information found.

What will be the possible reason for this failure? Where could I get more useful information about this failed task?

Thanks!