irule help to fix the error

Can you describe what you're trying to do, it looks like you're attempting to replace the host header but your formatting and syntax is muddled.

If just replacing host header you can try something like this

when HTTP_REQUEST { 
    if {[string tolower [HTTP::host]] contains "test1.com"} { 
        set newHost "test2.com"
        HTTP::header replace Host $newHost 
    } 
}