iRule error - bad IP address format (line x)

Here X-Forwarded-For Value is coming in String and you are changing to IP:Addr that the reason getting error.

TCL error: /parition1/BlockedCoun_XFF <HTTP_REQUEST> - bad IP address format (line 2)     invoked from within "whereis $client_ip country"
TCL error: /parition1/BlockedCoun_XFF <HTTP_REQUEST> - bad IP address format (line 3)     invoked from within "whereis $client_ip country"

Below iRule has not tested but try it

when HTTP_REQUEST {
    if { [HTTP::header exists "X-Forwarded-For"] } {
         set client_ip [HTTP::header insert X-Forwarded-For]
         log local0. "$client_ip"
         set fromCountry [whereis $client_ip country]
         if { ( [class match $fromCountry equals Blocked_Countries]) }{
                drop
        }
   }
}