rt-server: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/sbin/rt-server - from package: rt3

You may install the required package with command yum install {package}

Error in other languages:
rt-server: bevel nie gevind nie
rt-server: comandă negăsită
rt-server: comando não encontrado
rt-server: commande introuvable
rt-server: command not found
rt-server: käsku ei ole
rt-server: không tìm thấy lệnh
rt-server: komanda nerasta
rt-server: Kommando nicht gefunden.
rt-server: kommandot finns inte
rt-server: komut yok
rt-server: nie znaleziono polecenia
rt-server: níor aimsíodh an t-ordú
rt-server: no se encontró la orden
rt-server: no s'ha trobat l'ordre
rt-server: opdracht niet gevonden
rt-server: parancs nem található
rt-server: perintah tidak ditemukan
rt-server: príkaz nenájdený
rt-server: příkaz nenalezen
rt-server: команда не найдена
rt-server: командата не е открита
rt-server: 命令找不到
rt-server: コマンドが見つかりません