mrb: 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 Debian possible default full paths are:

/usr/bin/mrb - from package: mrb

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

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