dpkg-ruby: 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/dpkg-ruby - from package: ruby-debian

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

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