You are doing the filter behind the Format-Table, that will not work I'm afraid.
You can add it to the Where-clause you already have, something like this
Get-VM | Sort Name | Where-Object {$_.PowerState –eq “PoweredOn”} |
Get-CDDrive | Where {!$_.IsoPath} | Select Parent,IsoPath
The 2nd condition checks the property IsoPath.
A property that has a value is interpreted as $true, an empty property ($null) is considered $false.
And with the '!" (or -not) operator we negate the expression, so we look for objects that have an IsoPath property that is empty.