qmail-pw2u: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

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/sbin/qmail-pw2u - from package: qmail
/var/lib/qmail/bin/qmail-pw2u - from package: qmail

You may install the required package with command apt-get install {package}
Error in other languages:
qmail-pw2u: bevel nie gevind nie
qmail-pw2u: comandă negăsită
qmail-pw2u: comando não encontrado
qmail-pw2u: commande introuvable
qmail-pw2u: command not found
qmail-pw2u: käsku ei ole
qmail-pw2u: không tìm thấy lệnh
qmail-pw2u: komanda nerasta
qmail-pw2u: Kommando nicht gefunden.
qmail-pw2u: kommandot finns inte
qmail-pw2u: komut yok
qmail-pw2u: nie znaleziono polecenia
qmail-pw2u: níor aimsíodh an t-ordú
qmail-pw2u: no se encontró la orden
qmail-pw2u: no s'ha trobat l'ordre
qmail-pw2u: opdracht niet gevonden
qmail-pw2u: parancs nem található
qmail-pw2u: perintah tidak ditemukan
qmail-pw2u: príkaz nenájdený
qmail-pw2u: příkaz nenalezen
qmail-pw2u: команда не найдена
qmail-pw2u: командата не е открита
qmail-pw2u: 命令找不到
qmail-pw2u: コマンドが見つかりません