virt-customize: 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/virt-customize - from package: libguestfs-tools

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

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