Braces are required around expression when HTTP_REQUEST

you would want a fully compliant html page there complete with tags. Most use iFiles, which of course are not yet available to you on your version, but you can put the html in a variable in RULE_INIT.

when RULE_INIT {
  set static::sorry_page "..."
}
when HTTP_REQUEST {
  if { [active_members Test_Pool] < 1 } { 
    HTTP::respond 200 content $static::sorry_page Cache-Control "no-cache" Pragma "no-cache"
  }
}