db2x_docbook2man: command not found

Have You tried accessing the binary with the full path?

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH

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/db2x_docbook2man - from package: docbook2X

You may install the required package with command yum install {package}
Error in other languages:
db2x_docbook2man: bevel nie gevind nie
db2x_docbook2man: comandă negăsită
db2x_docbook2man: comando não encontrado
db2x_docbook2man: commande introuvable
db2x_docbook2man: command not found
db2x_docbook2man: käsku ei ole
db2x_docbook2man: không tìm thấy lệnh
db2x_docbook2man: komanda nerasta
db2x_docbook2man: Kommando nicht gefunden.
db2x_docbook2man: kommandot finns inte
db2x_docbook2man: komut yok
db2x_docbook2man: nie znaleziono polecenia
db2x_docbook2man: níor aimsíodh an t-ordú
db2x_docbook2man: no se encontró la orden
db2x_docbook2man: no s'ha trobat l'ordre
db2x_docbook2man: opdracht niet gevonden
db2x_docbook2man: parancs nem található
db2x_docbook2man: perintah tidak ditemukan
db2x_docbook2man: príkaz nenájdený
db2x_docbook2man: příkaz nenalezen
db2x_docbook2man: команда не найдена
db2x_docbook2man: командата не е открита
db2x_docbook2man: 命令找不到
db2x_docbook2man: コマンドが見つかりません