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 CentOS possible default full paths are:

/usr/bin/virtualenv - from package: python-virtualenv

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/virtualenv - from package: virtualenv

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

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