error after adding new irule

would you mind changing the existing irule to something like this?

root@(ve11a)(cfg-sync In Sync)(Active)(/Common)(tmos) list ltm rule xff
ltm rule xff {
    when HTTP_REQUEST_SEND {
  clientside {
    HTTP::header remove X-Forwarded-For
    HTTP::header insert X-Forwarded-For [substr [IP::client_addr] 0 "%"]
  }
}
}