Can not convert a VMX from vCenter server to a local vmx

I am trying to convent a vmx within vCenter server to a local vmx, i am trying with:

ovftool.exe "vi://user:password@my_vCenterserver/may_datacenter?ds=[my_datastore] my_vm/my_vm.vmx" C:\ovfs\my_vm.vmx

as the tool user guide Page 35, session "Converting a VM on ESXi or vCenter to an OVF" introduces.

But I got prompt as:

Opening VI source: vi://my_vCenterserver:443/

Error: Datacenter object not found

Completed with errors

But, i am sure the my_datacenter is INDEED exists.

Any suggestions what should i do?

Additional:

I even can NOT probe my VMX like this:

ovftool.exe vi://user:password@my_vCenterserver/my_datacenter/my_vm/my_vm.vmx

with Error:

Error: Locator does not refer to an object: vi://user:password@my_vCenterserver/my_datacenter/my_vm/my_vm.vmx

But I am very sure 'my_vmx' locates indeed there.