rb: 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/bin/rb - from package: lrzsz

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

On Debian possible default full paths are:

/usr/bin/rb - from package: lrzsz

You may install the required package with command apt-get install {package}

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