dh_virtualenv: command not found

Have You tried accessing the binary with the full path?

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/dh_virtualenv - from package: dh-virtualenv

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