docbook-xsl-update: 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/share/sgml/docbook/xsl-ns-stylesheets-1.75.2/tools/bin/docbook-xsl-update - from package: docbook5-style-xsl

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

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