dh_python-ply: 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/dh_python-ply - from package: python-ply

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

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