dtddiff2html: 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/dtddiff2html - from package: libsgml-dtdparse-perl

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

Error in other languages:
dtddiff2html: bevel nie gevind nie
dtddiff2html: comandă negăsită
dtddiff2html: comando não encontrado
dtddiff2html: commande introuvable
dtddiff2html: command not found
dtddiff2html: käsku ei ole
dtddiff2html: không tìm thấy lệnh
dtddiff2html: komanda nerasta
dtddiff2html: Kommando nicht gefunden.
dtddiff2html: kommandot finns inte
dtddiff2html: komut yok
dtddiff2html: nie znaleziono polecenia
dtddiff2html: níor aimsíodh an t-ordú
dtddiff2html: no se encontró la orden
dtddiff2html: no s'ha trobat l'ordre
dtddiff2html: opdracht niet gevonden
dtddiff2html: parancs nem található
dtddiff2html: perintah tidak ditemukan
dtddiff2html: príkaz nenájdený
dtddiff2html: příkaz nenalezen
dtddiff2html: команда не найдена
dtddiff2html: командата не е открита
dtddiff2html: 命令找不到
dtddiff2html: コマンドが見つかりません