Restore SQL Server database using Windows Powershell 3.0
You're going to have to get into that exception to see what the problem is. Powershell keeps track of errors for you in a system variable called $Error, which is a heap (i.e. index 0 is the most recent error). I usually do something like this:

$e = $error[0]
$e.Exception
$e.Exception.InnerException
$e.Exception.InnerException.InnerException
...


Until I find the real error. It could be a problem with the actual restore (i.e. database with that name already exists, bad paths for the physical files, etc). But until you get to the root of that exception you won't know!