umount.aufs: 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.aufs - from package: aufs-tools

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

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