twoftpd-drop: 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/twoftpd-drop - from package: twoftpd

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

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