Client authentication is set to require but not getting any log from this iRule

To accomplish what you're asking, I reworked the iRule a little bit to clean up the code a little bit. Only thing you may have to change is the [SSL::cert 0] to [SSL::cert 1] if you get an error, as I'm not sure if its a 0 or 1 based array. See if that helps.

when CLIENTSSL_CLIENTCERT {
     Debug flag
    set debug 1

    if {$debug >= 1} {
         Check if client presented a cert after it was requested/required
        if {[SSL::cert count] > 0} {        
             Loop through each cert and log the cert subject, issuer and serial number
            log local0. "[IP::client_addr]:[TCP::client_port]: subject=[X509::subject [SSL::cert 0]]"
        } else {
            log local0. "[IP::client_addr]:[TCP::client_port]: No client cert found!"
        }
    }
}