xml2rfc: 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/xml2rfc - from package: python-xml2rfc

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

Error in other languages:
xml2rfc: bevel nie gevind nie
xml2rfc: comandă negăsită
xml2rfc: comando não encontrado
xml2rfc: commande introuvable
xml2rfc: command not found
xml2rfc: käsku ei ole
xml2rfc: không tìm thấy lệnh
xml2rfc: komanda nerasta
xml2rfc: Kommando nicht gefunden.
xml2rfc: kommandot finns inte
xml2rfc: komut yok
xml2rfc: nie znaleziono polecenia
xml2rfc: níor aimsíodh an t-ordú
xml2rfc: no se encontró la orden
xml2rfc: no s'ha trobat l'ordre
xml2rfc: opdracht niet gevonden
xml2rfc: parancs nem található
xml2rfc: perintah tidak ditemukan
xml2rfc: príkaz nenájdený
xml2rfc: příkaz nenalezen
xml2rfc: команда не найдена
xml2rfc: командата не е открита
xml2rfc: 命令找不到
xml2rfc: コマンドが見つかりません