dtd2xsd: 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 CentOS possible default full paths are:

/usr/bin/dtd2xsd - from package: mono-devel

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/dtd2xsd - from package: mono-devel

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

Error in other languages:
dtd2xsd: bevel nie gevind nie
dtd2xsd: comandă negăsită
dtd2xsd: comando não encontrado
dtd2xsd: commande introuvable
dtd2xsd: command not found
dtd2xsd: käsku ei ole
dtd2xsd: không tìm thấy lệnh
dtd2xsd: komanda nerasta
dtd2xsd: Kommando nicht gefunden.
dtd2xsd: kommandot finns inte
dtd2xsd: komut yok
dtd2xsd: nie znaleziono polecenia
dtd2xsd: níor aimsíodh an t-ordú
dtd2xsd: no se encontró la orden
dtd2xsd: no s'ha trobat l'ordre
dtd2xsd: opdracht niet gevonden
dtd2xsd: parancs nem található
dtd2xsd: perintah tidak ditemukan
dtd2xsd: príkaz nenájdený
dtd2xsd: příkaz nenalezen
dtd2xsd: команда не найдена
dtd2xsd: командата не е открита
dtd2xsd: 命令找不到
dtd2xsd: コマンドが見つかりません