Irule error-Command not valid in the current scope

This maybe? But I think you'll want to do your "[clock format [expr ([clock seconds]+$expire_content_timeout)] -format "%a, %d %h %Y %T GMT" -gmt true]" after the if, set it as a variable and then place the variable inside the HTTP::header replace.

    when HTTP_REQUEST {
            if {[HTTP::method] eq "GET" and [HTTP::uri] eq "/"}{ 
                            set expire_content_timeout 300 
            } else { 
                            set expire_content_timeout [findclass [string tolower [string range [HTTP::path] [string last . [HTTP::path]] end]] static_cache_24hrs " "] 
            } 
    }

    when HTTP_RESPONSE { 
            if { $expire_content_timeout ne "" } { 
                            HTTP::header replace "Cache-Control" "max-age=$expire_content_timeout, public" HTTP::header replace "Expires" "[clock format [expr ([clock seconds]+$expire_content_timeout)] -format "%a, %d %h %Y %T GMT" -gmt true]" 
            } else {
                            HTTP::header replace "Cache-Control" "private"
                            HTTP::header replace "Expires" "-1"
            } 
    }