pg_virtualenv: 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/pg_virtualenv - from package: postgresql-common

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

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