virt-df: 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/virt-df - from package: libguestfs-tools-c

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

On Debian possible default full paths are:

/usr/bin/virt-df - from package: libguestfs-tools

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

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