iRule to insert cookie when there is external site cookie on the APM

Not sure why the error, may be can try replace instead and also some logging to see what so unique here:

when REWRITE_REQUEST_DONE {
    log local0. "Request URI = [HTTP::uri]"
    foreach x [HTTP::header names] {
       log local0. "Request header($x) = [HTTP::header $x]"
    }
    HTTP::header replace "Cookie" "SMSESSION=10"
}