virt-tar: 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-tar - from package: libguestfs-tools

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

On Debian possible default full paths are:

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

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

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