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 CentOS possible default full paths are:

/usr/lib64/xulrunner-devel-17.0.10/sdk/bin/ply - from package: xulrunner-devel
/usr/lib/xulrunner-devel-17.0.10/sdk/bin/ply - from package: xulrunner-devel

You may install the required package with command yum install {package}

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