xmldiff: 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/xmldiff - from package: xmldiff

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

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