docbook-2-html: 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/docbook-2-html - from package: sgml2x

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

Error in other languages:
docbook-2-html: bevel nie gevind nie
docbook-2-html: comandă negăsită
docbook-2-html: comando não encontrado
docbook-2-html: commande introuvable
docbook-2-html: command not found
docbook-2-html: käsku ei ole
docbook-2-html: không tìm thấy lệnh
docbook-2-html: komanda nerasta
docbook-2-html: Kommando nicht gefunden.
docbook-2-html: kommandot finns inte
docbook-2-html: komut yok
docbook-2-html: nie znaleziono polecenia
docbook-2-html: níor aimsíodh an t-ordú
docbook-2-html: no se encontró la orden
docbook-2-html: no s'ha trobat l'ordre
docbook-2-html: opdracht niet gevonden
docbook-2-html: parancs nem található
docbook-2-html: perintah tidak ditemukan
docbook-2-html: príkaz nenájdený
docbook-2-html: příkaz nenalezen
docbook-2-html: команда не найдена
docbook-2-html: командата не е открита
docbook-2-html: 命令找不到
docbook-2-html: コマンドが見つかりません