XMLValidator: 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/XMLValidator - from package: topp

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

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