mount.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/mount.aufs - from package: aufs-tools

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

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