umountpy: 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/umountpy - from package: mountpy

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

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