9umount: 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/9umount - from package: 9mount

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

Error in other languages:
9umount: bevel nie gevind nie
9umount: comandă negăsită
9umount: comando não encontrado
9umount: commande introuvable
9umount: command not found
9umount: käsku ei ole
9umount: không tìm thấy lệnh
9umount: komanda nerasta
9umount: Kommando nicht gefunden.
9umount: kommandot finns inte
9umount: komut yok
9umount: nie znaleziono polecenia
9umount: níor aimsíodh an t-ordú
9umount: no se encontró la orden
9umount: no s'ha trobat l'ordre
9umount: opdracht niet gevonden
9umount: parancs nem található
9umount: perintah tidak ditemukan
9umount: príkaz nenájdený
9umount: příkaz nenalezen
9umount: команда не найдена
9umount: командата не е открита
9umount: 命令找不到
9umount: コマンドが見つかりません