docbook2txt: 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/docbook2txt - from package: docbook-utils

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

On Debian possible default full paths are:

/usr/bin/docbook2txt - from package: docbook-utils

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

Error in other languages:
docbook2txt: bevel nie gevind nie
docbook2txt: comandă negăsită
docbook2txt: comando não encontrado
docbook2txt: commande introuvable
docbook2txt: command not found
docbook2txt: käsku ei ole
docbook2txt: không tìm thấy lệnh
docbook2txt: komanda nerasta
docbook2txt: Kommando nicht gefunden.
docbook2txt: kommandot finns inte
docbook2txt: komut yok
docbook2txt: nie znaleziono polecenia
docbook2txt: níor aimsíodh an t-ordú
docbook2txt: no se encontró la orden
docbook2txt: no s'ha trobat l'ordre
docbook2txt: opdracht niet gevonden
docbook2txt: parancs nem található
docbook2txt: perintah tidak ditemukan
docbook2txt: príkaz nenájdený
docbook2txt: příkaz nenalezen
docbook2txt: команда не найдена
docbook2txt: командата не е открита
docbook2txt: 命令找不到
docbook2txt: コマンドが見つかりません