dtd2rng: 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/dtd2rng - from package: mono-devel

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

On Debian possible default full paths are:

/usr/bin/dtd2rng - from package: mono-devel

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

Error in other languages:
dtd2rng: bevel nie gevind nie
dtd2rng: comandă negăsită
dtd2rng: comando não encontrado
dtd2rng: commande introuvable
dtd2rng: command not found
dtd2rng: käsku ei ole
dtd2rng: không tìm thấy lệnh
dtd2rng: komanda nerasta
dtd2rng: Kommando nicht gefunden.
dtd2rng: kommandot finns inte
dtd2rng: komut yok
dtd2rng: nie znaleziono polecenia
dtd2rng: níor aimsíodh an t-ordú
dtd2rng: no se encontró la orden
dtd2rng: no s'ha trobat l'ordre
dtd2rng: opdracht niet gevonden
dtd2rng: parancs nem található
dtd2rng: perintah tidak ditemukan
dtd2rng: príkaz nenájdený
dtd2rng: příkaz nenalezen
dtd2rng: команда не найдена
dtd2rng: командата не е открита
dtd2rng: 命令找不到
dtd2rng: コマンドが見つかりません