mruby: 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/mruby - from package: mruby

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

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