I'm not sure why that is not working, you can do the following for now.
if (!(Get-PSSnapin -Name "SwisSnapin" -ErrorAction SilentlyContinue))
{ Add-PSSnapin SwisSnapin -ErrorAction SilentlyContinue
}
$swisTarget = 'localhost'
$username="admin"
$password=""
$swis = Connect-Swis -host $swisTarget -UserName $username -Password $password
#Get the first alert that isn't acknowledged
$query = "SELECT TOP 1 AlertDefID AS DefinitionId, ActiveObject AS ObjectId, ObjectType FROM Orion.AlertStatus WHERE Acknowledged = 0"
$queryResult = Get-SwisData $swis $query
ForEach ($ActiveAlert in $queryResult)
{
[xml]$invokeResult $invokeResult = Invoke-SwisVerb $swis "Orion.AlertStatus" "Acknowledge" @(([xml]@( "<ArrayOfAlerto xmlns='http://schemas.solarwinds.com/2008/Orion'>", [string](%{ "<AlertInfo><DefinitionId>$($ActiveAlert.DefinitionId)</DefinitionId><ObjectType>$($ActiveAlert.ObjectType)</ObjectType><ObjectId>$($ActiveAlert.ObjectId)</ObjectId></AlertInfo>" } ), "</ArrayOfAlertInfo>" )).DocumentElement) $result = $invokeResult.ChildNodes.Value Write-Host "Acknowledge events returned $result"
}