Im having a similar issue.
When i try update the template i get a error regardless of the object i try update. The get command works fine and i see a template and id
$pool = Get-HVPool -PoolName IX_TESTING_20H2
$pool.automatedDesktopData.virtualCenterProvisioningSettings.virtualCenterProvisioningData.Template
$pool.AutomatedDesktopData.VirtualCenterNamesData.TemplatePath
Exception calling "Desktop_Update" with "3" argument(s): "ExceptionType : VMware.Hv.InvalidArgument
ErrorMessage : Invalid member name.
ParameterName : automatedDesktopData.virtualCenterProvisioningSettings.virtualCenterProvisioningData.Template"
At C:\Program Files\WindowsPowerShell\Modules\VMware.Hv.Helper\VMware.HV.Helper.psm1:6450 char:8
+ $desktop_helper.Desktop_Update($services,$item,$updates)
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : NotSpecified: (:) [], MethodInvocationException
+ FullyQualifiedErrorId : VimException
I have tried these commands
Set-HVPool -PoolName 'IT_TESTING' -Key 'automatedDesktopData.virtualCenterProvisioningSettings.virtualCenterProvisioningData.Template' -Value 'VmTemplate/MDcyMzg4OTMtNTk4Ni00OGIyLThkZDItNmE5YzA0MmQwNDUz/NjljNzkyMTItZjljNC00NjYwLTk3MjctMzJkOTc3NjZkNTZj/L0lYIFZESSBDTFVTVEVSL3ZtL0JVSUxEL1cxMF8yMEgyL1dpbjEwXzIwSDJfQmFzZV9NYXN0ZXIwMQ'
Set-HVPool -PoolName 'IT_TESTING' -Key 'automatedDesktopData.virtualCenterProvisioningSettings.virtualCenterProvisioningData.Template' -Value '/VDI CLUSTER/vm/BUILD/Win10_20H2_Base_Master01'
Any ideas?