Have you first established a connection to the SVM where the SMB share currently resides?
Also are you using the IP or the name which resolves to the LIF that has CIFS access? (Not the vserver management LIF)
$svm="name of the svm"
$clustermgmt="cluster mgmt lif"
$sourcevmdk="\\svmcifsname\share\first.vmdk"
$destinationVHDx="\\svmcifsname\share1\first.vhdx
$clustersvm=Connect-NcController $clustermgmt -vserver $svm -HTTPS
ConvertTo-NcVhdx -controller $clustersvm -SourceVmdk $sourceVMDK -DestinationVhd $destinationVHD
Cheers,
Rahul