Literal[1] newlines (line breaks) in XML attributes of XML text are seemingly not preserved when XML text is parsed into a DOM (Document Object Model); they are replaced with spaces.
Therefore, whatever code you put in your Executor attribute ends up as a single line, which is why you must ;, PowerShell's statement separator, to separate your statements; note the ; after Get-RDUserSession and note that the line breaks are purely for readability here:
<Action name="KillAllUsers" Type="Powershell" Executor='
$sessions = Get-RDUserSession;
foreach ( $session in $sessions ) {
Invoke-RDUserLogoff -HostServer $session.HostServer -UnifiedSessionID $session.UnifiedSessionId -Force
}
'></Action>
As for the error you saw:
Due to your code being placed all on a single line without statement separators, the foreach statement syntactically became an argument to the Get-RDUserSession command, which caused a syntax error; a simple way to provoke the error is:
# -> ERROR: "Unexpected token 'in' in expression or statement."
Get-Date foreach ($i in 1..2) { $i }
[1] By contrast, escaped line breaks (
 for LF-only newlines, 
 for CRLF newlines) are preserved: they are converted to literal newlines in memory (in the DOM), and reconverted to their escaped form during serialization, i.e. when converting the in-memory DOM to a string or saving it to a file.
