umount.nilfs2: 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.nilfs2 - from package: nilfs-tools

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

Error in other languages:
umount.nilfs2: bevel nie gevind nie
umount.nilfs2: comandă negăsită
umount.nilfs2: comando não encontrado
umount.nilfs2: commande introuvable
umount.nilfs2: command not found
umount.nilfs2: käsku ei ole
umount.nilfs2: không tìm thấy lệnh
umount.nilfs2: komanda nerasta
umount.nilfs2: Kommando nicht gefunden.
umount.nilfs2: kommandot finns inte
umount.nilfs2: komut yok
umount.nilfs2: nie znaleziono polecenia
umount.nilfs2: níor aimsíodh an t-ordú
umount.nilfs2: no se encontró la orden
umount.nilfs2: no s'ha trobat l'ordre
umount.nilfs2: opdracht niet gevonden
umount.nilfs2: parancs nem található
umount.nilfs2: perintah tidak ditemukan
umount.nilfs2: príkaz nenájdený
umount.nilfs2: příkaz nenalezen
umount.nilfs2: команда не найдена
umount.nilfs2: командата не е открита
umount.nilfs2: 命令找不到
umount.nilfs2: コマンドが見つかりません