umountavfs: 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/umountavfs - from package: avfs

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

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