guestunmount: 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/guestunmount - from package: libguestfs-tools

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

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