merb: 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/merb - from package: ruby-merb-core

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

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