xml_pp: 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/bin/xml_pp - from package: perl-XML-Twig

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

On Debian possible default full paths are:

/usr/bin/xml_pp - from package: xml-twig-tools

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

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