I am attempting to Set-Acl
on a Computer Object in AD. Firstly I get the ACL using:
$acl = (Get-Acl AD:\\\'CN=Tester1,OU=Ou1,OU=OU2,OU=OU3,DC=Co
ACE for AD objects you must create with System.DirectoryServices.ActiveDirectoryAccessRule object instead of System.Security.AccessControl.FileSystemAccessRule.
Good description and example is here: Add Object Specific ACEs using Active Directory Powershell
ActiveDirectory isn't a filesystem. You must create a new ACE for an AD object as an ActiveDirectoryAccessRule.
$path = "AD:\CN=Tester1,OU=Ou1,OU=OU2,OU=OU3,DC=Contoso,DC=com"
$acl = Get-Acl -Path $path
$ace = New-Object Security.AccessControl.ActiveDirectoryAccessRule('DOMAIN\Computername','FullControl')
$acl.AddAccessRule($ace)
Set-Acl -Path $path -AclObject $acl