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

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

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