sippasswd: 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/sippasswd - from package: sipwitch

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

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