So i have to apologize for beeing stupid 
the tutorial said to do exactly what it is written How to setup a RedM Server
but I though I was a genius to create a bat file located in the root folder (the one that contains server-file and server-data directories)
but to fix my issue, the command line to start the server has to be maid from the server-data directory and must exactly be :
…\server-files\FXServer.exe +exec server.cfg +set gamename rdr3
then everything was working.
I hope my mistake will help others…