mongrel2: 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/mongrel2 - from package: mongrel2-core

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

Error in other languages:
mongrel2: bevel nie gevind nie
mongrel2: comandă negăsită
mongrel2: comando não encontrado
mongrel2: commande introuvable
mongrel2: command not found
mongrel2: käsku ei ole
mongrel2: không tìm thấy lệnh
mongrel2: komanda nerasta
mongrel2: Kommando nicht gefunden.
mongrel2: kommandot finns inte
mongrel2: komut yok
mongrel2: nie znaleziono polecenia
mongrel2: níor aimsíodh an t-ordú
mongrel2: no se encontró la orden
mongrel2: no s'ha trobat l'ordre
mongrel2: opdracht niet gevonden
mongrel2: parancs nem található
mongrel2: perintah tidak ditemukan
mongrel2: príkaz nenájdený
mongrel2: příkaz nenalezen
mongrel2: команда не найдена
mongrel2: командата не е открита
mongrel2: 命令找不到
mongrel2: コマンドが見つかりません