docbook2pdf: 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/docbook2pdf - from package: docbook-utils-pdf

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

On Debian possible default full paths are:

/usr/bin/docbook2pdf - from package: docbook-utils

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

Error in other languages:
docbook2pdf: bevel nie gevind nie
docbook2pdf: comandă negăsită
docbook2pdf: comando não encontrado
docbook2pdf: commande introuvable
docbook2pdf: command not found
docbook2pdf: käsku ei ole
docbook2pdf: không tìm thấy lệnh
docbook2pdf: komanda nerasta
docbook2pdf: Kommando nicht gefunden.
docbook2pdf: kommandot finns inte
docbook2pdf: komut yok
docbook2pdf: nie znaleziono polecenia
docbook2pdf: níor aimsíodh an t-ordú
docbook2pdf: no se encontró la orden
docbook2pdf: no s'ha trobat l'ordre
docbook2pdf: opdracht niet gevonden
docbook2pdf: parancs nem található
docbook2pdf: perintah tidak ditemukan
docbook2pdf: príkaz nenájdený
docbook2pdf: příkaz nenalezen
docbook2pdf: команда не найдена
docbook2pdf: командата не е открита
docbook2pdf: 命令找不到
docbook2pdf: コマンドが見つかりません