sgml2xml-isoent: 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/sgml2xml-isoent - from package: docbook2X

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

On Debian possible default full paths are:

/usr/bin/sgml2xml-isoent - from package: docbook2X

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

Error in other languages:
sgml2xml-isoent: bevel nie gevind nie
sgml2xml-isoent: comandă negăsită
sgml2xml-isoent: comando não encontrado
sgml2xml-isoent: commande introuvable
sgml2xml-isoent: command not found
sgml2xml-isoent: käsku ei ole
sgml2xml-isoent: không tìm thấy lệnh
sgml2xml-isoent: komanda nerasta
sgml2xml-isoent: Kommando nicht gefunden.
sgml2xml-isoent: kommandot finns inte
sgml2xml-isoent: komut yok
sgml2xml-isoent: nie znaleziono polecenia
sgml2xml-isoent: níor aimsíodh an t-ordú
sgml2xml-isoent: no se encontró la orden
sgml2xml-isoent: no s'ha trobat l'ordre
sgml2xml-isoent: opdracht niet gevonden
sgml2xml-isoent: parancs nem található
sgml2xml-isoent: perintah tidak ditemukan
sgml2xml-isoent: príkaz nenájdený
sgml2xml-isoent: příkaz nenalezen
sgml2xml-isoent: команда не найдена
sgml2xml-isoent: командата не е открита
sgml2xml-isoent: 命令找不到
sgml2xml-isoent: コマンドが見つかりません