dtddiff: 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/dtddiff - from package: libsgml-dtdparse-perl

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

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