Get-CIVAppNetwork -VApp $NewVapp -name *mgmt-192* | Set-CIVAppNetwork -FirewallEnabled:$false -NatEnabled:$true -ParentOrgNetwork $Parentnetwork
What is the content of the variable "$Parentnetwork" ?
As far as I know, the option "-ParentOrgNetwork" requires an object from type "OrgNetwork". Not a string. So, if you only specify the name in this variable it won't work.
If that's the case with you, something like this could work:
$Parentnetwork = Get-OrgNetwork -Name 'orgNetworkName'
Get-CIVAppNetwork -VApp $NewVapp -name *mgmt-192* | Set-CIVAppNetwork -FirewallEnabled:$false -NatEnabled:$true -ParentOrgNetwork $Parentnetwork