7,023 questions
You have 2 PS statements on one line and you have not separated them with the ";" character.
Put the write-output on a new line.
$secidentifier = New-Object System.Security.Principal.SecurityIdentifier("S-1-5-21-7151733594-696023681-915538951-9220120173")
$username = $secidentifier.Translate([System.Security.Principal.NTAccount]).Value
Write-Output $username