umount.ploop: 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:

/sbin/umount.ploop - from package: ploop

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

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