Thanks @mpotti , looks like I have tweaked the sailpoint lines, so it was giving me the error:
I tweaked it to
$sReader = New-Object System.IO.StringReader([System.String]$requestString);
while it should be
$sReader = New-Object System.IO.StringReader([System.String]$env:Request);
not sure why I did that, but after reverting it back to what SP documentation has, I was able to do the operation through powershell.
Thanks for the response though, Mark…