mirb: 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/mirb - from package: mruby

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

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